-
10
Small Streams and WetlandsProvide Beneficial
EcosystemServices
Natural processes that occur in smallstreams and wetlands
provide humanswith a host of benefits, including flood
control,maintenance of water quantity and quality, andhabitat for a
variety of plants and animals. Forheadwater streams and wetlands to
provide ecosys-tem services that sustain the health of our
nation’swaters, the hydrological, geological and
biologicalcomponents of stream networks must be intact.
Small Streams and WetlandsProvide Natural Flood ControlFloods
are a natural part of every river. In timespast, waters of the
Mississippi River routinelyovertopped its banks. Floodwaters
carried thesediment and nutrients that made theMississippi Delta’s
soil particularly suitable foragriculture. But floods can also
destroy farms,houses, roads and bridges.
When small streams and wetlands are in their nat-ural state,
they absorb significant amounts of rain-water, runoff and snowmelt
before flooding.However, when a landscape is altered, such as by
alandslide or large forest fire or a housing develop-ment, the
runoff can exceed the absorption capac-ity of small streams.
Moreover, the power ofadditional water coursing through a channel
canchange the channel itself. Humans often alter bothlandscape and
stream channels in ways that resultin larger and more frequent
floods downstream.
A key feature of streams and rivers is their shape.Unlike a
concrete drainage ditch, a naturalstreambed does not present a
smooth surface forwater flow. Natural streambeds are rough andbumpy
in ways that slow the passage of water.Particularly in small narrow
streams, friction pro-duced by a stream’s gravel bed, rocks, and
dams ofleaf litter and twigs slows water as it moves down-
stream. Slower moving water is more likely to seepinto a
stream’s natural water storage system-its bedand banks-and to
recharge groundwater. Slowermoving water also has less power to
erode streambanks and carry sediment and debris downstream.
In watersheds that are not carefully protectedagainst impacts of
land development, stream chan-nels often become enlarged and
incised fromincreased runoff. Changed channels send waterdownstream
more quickly, resulting in moreflooding. For example, after forests
and prairies inWisconsin watersheds were converted to agricul-tural
fields, the size of floods increased. Thischange in land use had
altered two parts of theriver systems’ equation: the amount of
runoff andshape of the stream channel. Cultivation destroyedthe
soil’s natural air spaces that came from wormburrows and plant
roots. The resulting collapse ofthe soil caused more rainfall to
run off into streamsinstead of soaking into the ground. Additional
sur-face runoff then altered the stream channels,thereby increasing
their capacity to carry large vol-umes of water quickly downstream.
These largervolumes flow downstream at much higher velocity,rather
than soaking into the streambed.
Urbanization has similar effects; paving previ-ously-vegetated
areas leads to greater storm runoff,which changes urban stream
channels and ulti-mately sends water more quickly
downstream.Covering the land with impermeable surfaces,such as
roofs, roads, and parking lots, can increaseby several times the
amount of runoff from a rain-storm. If land uses change near
headwater streams,effects are felt throughout the stream network.
Inan urban setting, runoff is channeled into stormsewers, which
then rapidly discharge large volumesof water into nearby streams.
The additional watercauses the stream to pick up speed, because
deeper
A headwater stream
channel near Toledo, OH
relocated to accommodate
development.
Photo courtesy of
Marshal A. Moser
-
11
water has less friction with the streambed. Thefaster the water
moves, the less it can soak into thestreambed and banks. Faster
water also erodeschannel banks and beds, changing the shape of
achannel. The effect is magnified downstream,because larger rivers
receive water from tens, some-times hundreds, of small headwater
basins. Whensuch changes are made near headwater streams,downstream
portions of the stream network expe-rience bigger and more frequent
flooding.
As regions become more urbanized,humans intentionally alter
manynatural stream channels by replacingthem with storm sewers and
otherartificial conduits. When larger,smoother conduits are
substitutedfor narrow, rough-bottomed naturalstream channels, flood
frequencyincreases downstream. For example,three decades of growth
in stormsewers and paved surfaces aroundWatts Branch Creek,
Marylandmore than tripled the number offloods and increased average
annualflood size by 23 percent.
Small Streams and WetlandsMaintain Water SuppliesHeadwater
systems play a crucial role in ensuringa continual flow of water to
downstream freshwa-ter ecosystems. Water in streams and rivers
comesfrom several sources: water held in the soil, runofffrom
precipitation, and groundwater. Watermoves between the soil,
streams and groundwater.Wetlands, even those without any obvious
surfaceconnection to streams, are also involved in suchexchanges by
storing and slowly releasing waterinto streams and groundwater,
where it laterresurfaces at springs. Because of these
interactions,groundwater can contribute a significant portionof
surface flow in streams and rivers; conversely,surface waters can
also recharge groundwater. Ifconnections between soil, water,
surface waters,and groundwater are disrupted, streams, rivers,and
wells can run dry. Two-thirds of Americansobtain their drinking
water from a water systemthat uses surface water. The remaining
one-thirdof the population relies on groundwater sources.
The quality and amount of water in both of thesesources respond
to changes in headwater streams.
USGS estimates that, on average, from 40 to 50percent of water
in streams and larger rivers comesfrom groundwater. In drier
regions or during dryseasons, as much as 95 percent of a stream’s
flowmay come from groundwater. Thus, the rechargeprocess that
occurs in unaltered headwaterstreams and wetlands both moderates
down-stream flooding in times of high water and main-
tains stream flow during dry seasons.
Headwater streams and wetlands havea particularly important role
to playin recharge. These smallest upstreamcomponents of a river
network havethe largest surface area of soil in con-tact with
available water, thereby pro-viding the greatest opportunity
forrecharge of groundwater. Moreover,water level in headwater
streams isoften higher than the water table,allowing water to flow
through thechannel bed and banks into soil andgroundwater. Such
situations occurwhen water levels are high, such as
during spring snowmelt or rainy seasons. Duringdry times, the
situation in some reaches of thestream network, particularly those
downstream,may reverse, with water flowing from the soil
andgroundwater through the channel banks and bedinto the stream.
This exchange of water from thesoil and groundwater into the stream
maintainsstream flow. However, if land-use changes increasethe
amount of precipitation that runs off into astream rather than
soaking into the ground, therecharge process gets short-circuited.
This increasedvolume of stream water flows rapidly downstreamrather
than infiltrating into soil and groundwater.The consequence is less
overall groundwaterrecharge, which often results in less water
instreams during drier seasons.
Therefore, alteration of small streams and wetlandsdisrupts the
quantity and availability of water in astream and river system.
Protecting headwaterstreams and wetlands is important for
maintainingwater levels needed to support everything from fishto
recreational boating to commercial ship traffic.
“ALTERATION OF
SMALL STREAMS
AND WETLANDS
DISRUPTS THE
QUANTITY AND
AVAILABILITY OF
WATER IN A
STREAM AND
RIVER SYSTEM.”
-
12
Small Streams and Wetlands TrapExcess SedimentHeadwater systems
retain sediment. Like the flowof water, movement of sediment occurs
through-out a river network. Thus, how a watershed ismanaged and
what kinds of land uses occur therehave substantial impact on the
amount of sedi-ment delivered to larger rivers downstream.Increased
sediment raises water purification costsfor municipal and
industrial users, requires exten-sive dredging to maintain
navigational channels,and degrades aquatic habitats. Intact
headwaterstreams and wetlands can modulate the amount ofsediment
transported to downstream ecosystems.
Runoff from rain, snowmelt andreceding floodwaters can wash
soil,leaves and twigs into streams, wherethe various materials get
broken upinto smaller particles or settle out. Ifnatural vegetation
and soil cover aredisturbed by events and activitiessuch as fires,
farming or construc-tion, runoff increases, washingmore materials
into streams. At thesame time, the increased velocityand volume of
water in a streamcause erosion within the streambedand banks
themselves, contributingadditional sediment to the streamsystem.
Moreover, the faster, fullerstream can carry more and largerchunks
of sediment further downstream.
One study found that land disturbances such asurban construction
can, at minimum, double theamount of sediment entering headwater
streamsfrom a watershed. A Pennsylvania study showedhow, as a
160-acre headwater watershed becamemore urbanized, channel erosion
of a quarter-mile stretch of stream generated 50,000 addi-tional
cubic feet of sediment in one year-enoughto fill 25 moderate-sized
living rooms. In a non-urban watershed of the same size, it would
takefive years to generate the same amount of sedi-ment. Such
studies demonstrate that landscapechanges such as urbanization or
agriculture, par-ticularly without careful protection of
headwaterstreams and their riparian zones, may cause manytimes more
sediment to travel downstream.
EXCESS SEDIMENT IN DOWNSTREAMECOSYSTEMS COSTS MONEYKeeping
excess sediment out of downstream riversand lakes is one ecosystem
service intact smallstreams and wetlands provide. Once
sedimentmoves further downstream, it becomes an expen-sive problem.
Too much sediment can fill up reser-voirs and navigation channels,
damage commercialand sport fisheries, eliminate recreation spots,
harmaquatic habitats and their associated plants and ani-mals, and
increase water filtration costs.
Additional sediment damages aquatic ecosys-tems. Sediment
suspended in the water makes itmurkier; as a result, underwater
plants no longer
receive enough light to grow. Fishthat depend on visual signals
tomate may be less likely to spawn inmurky water, thereby reducing
fishpopulations. High levels of sedi-ment suspended in water can
evencause fish kills. Even as it settles tothe bottom, sediment
continues tocause problems because it fills theholes between gravel
and stonesthat some animals call home,smothers small organisms
thatform the basis of many food webs,and can also smother fish
eggs.
Getting rid of sediment is expensive.For example, keeping
BaltimoreHarbor navigable costs $10 to $11.5
million annually to dredge and dispose of sedimentthe Patapsco
River deposits in the harbor.
SMALL STREAMS AND WETLANDS RETAINSEDIMENTHeadwater streams and
wetlands typically trap andretain much of the sediment that washes
into them.The faster the water travels, the larger the particles
itcan carry. So, natural obstructions in small streams-rocks,
downed logs, or even just a bumpy streambottom-slow water and cause
sediment to settle outof the water column. Wetlands, whether or not
theyhave a surface connection to a nearby stream, areoften areas
where runoff slows and stops, droppingany debris the water may be
carrying. Because head-water streams represent 75 percent or more
of totalstream length in a stream network, such streams andtheir
associated wetlands retain a substantial
“INTACT HEADWATER
STREAMS AND
WETLANDS CAN
MODULATE THE
AMOUNT OF
SEDIMENT
TRANSPORTED TO
DOWNSTREAM
ECOSYSTEMS.”
-
13
amount of sediment, preventing it from flowinginto larger rivers
downstream.
Even ephemeral streams can retain significantamounts of
sediment. Such small headwaterstreams expand and contract in
response to heavyrains. During expansion, a stream flows over
whatwas a dry or damp streambed. Most of the water atthe leading
edge of a growing stream, called the“trickle front,” soaks into the
streambed and doesnot carry sediment downstream. In a small
water-shed near Corvallis, Oregon, researchers found that60 to 80
percent of sediment generated from forestroads traveled less than
250 feet downstream beforesettling out in stream pools. Headwater
streams canstore sediment for long periods of time: research
inOregon’s Rock Creek basin found that headwaterstreams could
retain sediment for 114 years.
Natural Cleansing Ability ofSmall Streams and WetlandsProtects
Water Quality Materials that wash into streams include every-thing
from soil, leaves and dead insects to runofffrom agricultural
fields and animal pastures.One of the key ecosystem services that
streamnetworks provide is the filtering and processingof such
materials. Healthy aquatic ecosystemscan transform natural
materials like animal dungand chemicals such as fertilizers into
less harm-ful substances. Small streams and their
associatedwetlands play a key role in both storing andmodifying
potential pollutants, ranging fromchemical fertilizers to rotting
salmon carcasses,in ways that maintain downstream water
quality.
EXCESS NUTRIENTS CAUSE PROBLEMS INRIVERS AND LAKESInorganic
nitrogen and phosphorus, the mainchemicals in agricultural
fertilizers, are essentialnutrients not just for plants, but for
all livingorganisms. However, in excess or in the wrongproportions,
these chemicals can harm naturalsystems and humans.
In freshwater ecosystems, eutrophication, theenriching of waters
by excess nitrogen and phos-phorus, reduces water quality in
streams, lakes, estu-aries and other downstream waterbodies.
Oneobvious result is the excessive growth of algae. More
algae clouds previously clear streams, such as thosefavored by
trout. In addition to reducing visibility,algal blooms reduce the
amount of oxygen dissolvedin the water, sometimes to a degree that
causes fishkills. Fish are not the only organisms harmed: someof
the algae species that grow in eutrophic watersgenerate tastes and
odors or are toxic, a clear prob-lem for stream systems that supply
drinking waterfor municipalities. In addition, increased
nitrogencan injure people and animals. Excess nitrogen inthe form
called nitrate in drinking water has beenlinked to “blue baby
disease” (methemoglobinemia)in infants and also has toxic effects
on livestock.
HEADWATER STREAMS TRANSFORM ANDSTORE EXCESS NUTRIENTSHeadwater
streams and associated wetlands bothretain and transform excess
nutrients, thereby pre-venting them from traveling downstream.
Physical,chemical and biological processes in headwaterstreams
interact to provide this ecosystem service.
Compared with larger streams and rivers, smallstreams,
especially shallow ones, have more waterin physical contact with a
stream channel.Therefore, the average distance traveled by a
parti-cle before it is removed from the water column isshorter in
headwater streams than in larger ones. Astudy of headwater streams
in the southernAppalachian Mountains found that both phos-phorus
and the nitrogen-containing compoundammonium traveled less than 65
feet downstreambefore being removed from the water.
Stream networks filter and
process everything from
leaves and dead insects to
runoff from agricultural
fields and animal pastures.
Without such processing,
algal blooms can ruin living
conditions for fish and the
quality of drinking water.
Here, algae overtakes a lake
in Iowa. Photo courtesy of
Lynn Betts, USDA NRCS
-
14
In headwater streams and wetlands, more water is indirect
contact with the streambed, where most pro-cessing takes place.
Bacteria, fungi and other microor-ganisms living on the bottom of a
stream consumeinorganic nitrogen and phosphorus and convert
theminto less harmful, more biologically beneficial com-pounds. A
mathematical model based on research in14 headwater streams
throughout the U.S. shows that64 percent of inorganic nitrogen
entering a smallstream is retained or transformed within 1,000
yards.
Channel shape also plays a role in transformingexcess nutrients.
Studies in Pennsylvania have shownthat when the forest surrounding
headwaters isreplaced by meadows or lawns, increased
sunlightpromotes growth of grasses along stream banks. Thegrasses
trap sediments, create sod, and narrow thestream channel to
one-third of theoriginal width. Such narrowingreduces the amount of
streambedavailable for microorganisms thatprocess nutrients. As a
result, nitrogenand phosphorus travel downstreamfive to ten times
farther, increasingrisks of eutrophication.
Streams do not have to flow year-round to make significant
contribu-tions to water quality. Fertilizers andother pollutants
enter stream sys-tems during storms and other timesof high runoff,
the same times thatephemeral and intermittent streamsare most
likely to have water and process nutrients.Federal, state and local
programs spend consider-able sums of money to reduce non-point
sourceinputs of nutrients because they are a major threatto water
quality. One principal federal program,the EPA’s 319 cost-share
program, awarded morethan $1.3 billion between 1990 and 2001 to
statesand territories for projects to control non-pointpollution.
Failure to maintain nutrient removalcapacity of ephemeral and
intermittent streamsand wetlands would undermine these efforts.
Wetlands also remove nutrients from surface waters.Several
studies of riparian wetlands have found thatthose associated with
the smallest streams to be mosteffective in removing nutrients from
surface waters.For example, headwater wetlands comprise 45
percentof all wetlands able to improve water quality in four
Vermont watersheds. Another study found that wet-lands
associated with first-order streams are responsi-ble for 90 percent
of wetland phosphorus removal ineight northeastern watersheds. Such
studies demon-strate that riparian wetlands, especially those
associ-ated with small streams, protect water quality.
As land is developed, headwater streams are oftenfilled or
channeled into pipes or paved waterways,resulting in fewer and
shorter streams. For example,as the Rock Creek watershed in
Maryland wasurbanized, more than half of the stream channel
net-work was eliminated. In even more dramatic fash-ion, mining
operations in the mountains of centralAppalachia have removed
mountain tops and filledvalleys, wiping out entire headwater stream
net-works. From 1986 to 1998, more than 900 miles of
streams in central Appalachia wereburied, more than half of them
inWest Virginia.
If headwater streams and wetlands aredegraded or filled, more
fertilizerapplied to farm fields or lawns reacheslarger downstream
rivers. These largerrivers process excess nutrients from
fer-tilizer much more slowly than smallerstreams. Losing the
nutrient retentioncapacity of headwater streams wouldcause
downstream waterbodies to con-tain higher concentrations of
nitrogenand phosphorus. A likely consequenceof additional nutrients
would be the
further contamination and eutrophication of down-stream rivers,
lakes, estuaries and such waters as theGulf of Mexico.
Natural Recycling in HeadwaterSystems Sustains
DownstreamEcosystemsRecycling organic carbon contained in the
bodiesof dead plants and animals is a crucial ecosystemservice.
Ecological processes that transform inor-ganic carbon into organic
carbon and recycleorganic carbon are the basis for every food web
onthe planet. In freshwater ecosystems, much of therecycling
happens in small streams and wetlands,where microorganisms
transform everything fromleaf litter and downed logs to dead
salamandersinto food for other organisms in the aquatic foodweb,
including mayflies, frogs and salmon.
“IF HEADWATER
STREAMS AND
WETLANDS ARE
DEGRADED OR
FILLED, MORE
FERTILIZER APPLIED
TO FARM FIELDS OR
LAWNS REACHES
LARGER DOWN-
STREAM RIVERS.”
-
15
Like nitrogen and phosphorus, carbon is essentialto life but can
be harmful to freshwater ecosystemsif it is present in excess or in
the wrong chemicalform. If all organic material received by
headwaterstreams and wetlands went directly downstream,the glut of
decomposing material could depleteoxygen in downstream rivers,
thereby damagingand even killing fish and other aquatic life.
Theability of headwater streams to transform organicmatter into
more usable forms helps maintainhealthy downstream ecosystems.
HEADWATER STREAM SYSTEMS STORE ANDTRANSFORM EXCESS ORGANIC
MATTERIntact headwater systems both store and processorganic matter
in ways that modulate the release ofcarbon to downstream lakes
andrivers. Headwater systems receive largeamounts of organic
matter, which canbe retained and transformed intomore palatable
forms through decom-position processes. This organic mat-ter is
anything of biological origin thatfalls into, washes into or dies
in astream. Plant parts, such as leaves,twigs, stems and larger
bits of woodydebris, are the most common of theseitems. Another
source of organicmaterial is dead stream organisms,such as bits of
dead algae and bacteriaor bodies of insects and even largeranimals.
Waste products of plants and animals alsoadd organic carbon to
water. Water leaches dissolvedorganic carbon from organic materials
in a streamand watershed like tea from a tea bag.
Much of the organic matter that enters headwatersystems remains
there instead of continuing down-stream. One reason is that the
material often entersheadwater streams as large pieces, such as
leaves andwoody debris, that are not easily carried down-stream. In
addition, debris dams that accumulate inheadwater streams block the
passage of materials.One study found four times more organic
matteron the bottoms of headwater streams in forestedwatersheds
than on the bottoms of larger streams.
Another reason material stays in headwater streams isthat food
webs in small streams and wetlands processorganic matter
efficiently. Several studies have foundthat headwater streams are
far more efficient at trans-forming organic matter than larger
streams. For exam-
ple, one study showed that, for a given length ofstream, a
headwater stream had an eight-fold higherprocessing efficiency than
a fourth-order channeldownstream. Microorganisms in headwater
streamsystems use material such as leaf litter and otherdecomposing
material for food and, in turn, becomefood for other organisms. For
example, fungi thatgrow on leaf litter become nutritious food for
inverte-brates that make their homes on the bottom of astream,
including mayflies, stoneflies and caddis flies.These animals
provide food for larger animals, includ-ing birds such as
flycatchers and fish such as trout.
HEADWATER SYSTEMS SUPPLY FOOD FORDOWNSTREAM ECOSYSTEMSThe
organic carbon released by headwater streams
provides key food resources for down-stream ecosystems.
Headwaterecosystems control the form, qualityand timing of carbon
supply down-stream. Although organic matteroften enters headwaters
in largeamounts, such as when leaves fall inautumn or storm runoff
carries debrisinto the stream, those leaves anddebris are processed
more slowly. As aresult, carbon is supplied to down-stream food
webs more evenly over alonger period of time. Forms of car-bon
delivered range from dissolvedorganic carbon that feeds
microor-
ganisms to the drifting insects such as mayflies andmidges that
make ideal fish food. Such insects arethe preferred food of fish
such as trout, char andsalmon. One study estimated that fishless
headwa-ter streams in Alaska export enough drifting insectsand
other invertebrates to support approximatelyhalf of the fish
production in downstream waters.
Processed organic matter from headwater streamsfuels aquatic
food webs from the smallest streams tothe ocean. Only about half of
all first-order streamsdrain into second-order streams; the other
half feeddirectly into larger streams or directly into estuariesand
oceans, thus delivering their carbon directly tothese larger
ecosystems. The health and productivityof downstream ecosystems
depends on processedorganic carbon-ranging from dissolved organic
carbonto particles of fungus, and leaf litter to mayflies
andstoneflies-delivered by upstream headwater systems.
“THE ABILITY OF
HEADWATER STREAMS
TO TRANSFORM
ORGANIC MATTER
INTO MORE USABLE
FORMS HELPS
MAINTAIN HEALTHY
DOWNSTREAM
ECOSYSTEMS.”
-
16
Headwater Streams MaintainBiological Diversity
HEADWATER HABITATS ARE DIVERSEHeadwater streams are probably the
most variedof all running-water habitats; they range fromicy-cold
brooks tumbling down steep, boulder-filled channels to outflows
from desert springsthat trickle along a wash for a short
distancebefore disappearing into sand. As such, headwa-ter systems
offer an enormous array of habitatsfor plant, animal and microbial
life.
This variation is due to regional differences inclimate,
geology, land use and biology. Forexample, streams in limestone or
sandy regionshave very steady flow regimes compared withthose
located in impermeable shale or clay soils.Plants or animals found
only in certain regionscan also lend a distinctive character to
headwaterstreams. Regionally important riparian plants,such as
alder and tamarisk, exercise a stronginfluence on headwater
streams. Headwaterstreams in regions with beavers are vastly
differ-ent from those in regions without beavers.
Environmental conditions change throughout astream network. In
wet regions, streams grow largerand have wider channels, deeper
pools for shelter,and more permanent flow as they move down-stream.
In arid regions and even humid regionsduring dry periods, headwater
streams may becomesmaller downstream as water evaporates or
soaksinto a streambed. Because marked changes in envi-ronmental
conditions can occur over very short dis-tances, conditions
required by a headwater speciesmay exist for as little as 100 yards
of stream.Consequently, local populations of a species mayextend
over just a short distance, particularly inspring-fed headwaters
with sharp changes in envi-ronmental conditions along the length of
a stream.
With this variety of influences, headwaterstreams present a rich
mosaic of habitats, eachwith its own characteristic community of
plants,animals, and microorganisms.
HEADWATER SYSTEMS SUPPORT A DIVERSEARRAY OF ANIMALS AND
PLANTSThere has never been a complete inventory ofthe inhabitants
in even a single headwaterstream, much less surveys across many
types ofheadwaters that would permit a thorough under-standing of
biodiversity in headwater streams.Nevertheless, it is clear that
individual headwaterstreams support hundreds to thousands
ofspecies, ranging from bacteria to bats.
The species in a typical headwater stream includebacteria,
fungi, algae, higher plants, invertebrates,fish, amphibians, birds
and mammals. Headwaterstreams are rich feeding grounds. Large
amounts ofleaves and other organic matter that fall or blowinto
streams, the retention of organic matter in a
Top right: A hydrobiid snail
[Pyrgulopsis robusta] found
in the headwaters of the
Snake River in Wyoming.
Photo courtesy of
Dr. Robert Hershler
Center: Caddis flies and
other aquatic insects spend
their larval stage in
streams, feeding on the
algae, vegetation and
decaying plant matter. The
Brachycentris, a caddis fly
found in headwater
streams of eastern North
America, constructs a
protective case out of twigs,
leaves and other debris.
Photo courtesy of
David H. Funk
Bottom: American
dippers rely on headwater
streams for sustenance,
walking along stream bot-
toms and feeding on insect
larvae and crustaceans
among the rocks of the
streambed. This American
dipper was photographed at
Tanner’s Flat, just east of Salt
Lake City. Photo courtesy of
Pomera M. France
Top left: Populations of the
ellipse mussel
(Venustaconcha ellipsi-
formis) have disappeared
from many of its native
Midwestern headwaters.
Photo courtesy of Kevin
Cummings, Illinois Natural
History Survey
-
17
channel or debris dams, and the high rates of plantand algal
growth in unshaded headwaters all supplyfood sources for animals
such as caddis flies, snailsand crustaceans. These animals become
food forpredators such as fish, salamanders, crayfish, birdsand
mammals, which, in turn, become prey forlarger animals, including
herons, raccoons andotters. Many widespread species also use
headwa-ters for spawning sites, nursery areas, feeding areas,and
travel corridors. Thus, headwater habitats areimportant to species
like otters, flycatchers, andtrout, even though these species are
not restrictedto headwaters. The rich resource base that
headwa-ters provide causes the biotic diversity of headwaterstreams
to contribute to the productivity of bothlocal food webs and those
farther downstream.
Diversity of headwater systems results in diverseheadwater
plants and animals. Many of thesespecies are headwater specialists
and are mostabundant in or restricted to headwaters. Forexample,
water shrews live along small, coolstreams, feed on aquatic
invertebrates, and spendtheir entire lives connected to headwater
streams.Because different headwaters harbor differentspecies, the
number of headwater-dependentspecies across North America is far
greater thanthe number of species in any one headwater.
Headwater specialists often have small geographicranges. These
species, many of which are imperiled,include: species of minnows,
darters, and topmin-nows in southeastern springs and brooks;
aquaticsnails in spring-fed headwaters in the Great Basin,the
Southeast, Florida, and the Pacific Northwest;crayfish in small
streams from Illinois andOklahoma to Florida; and salamanders and
tailedfrogs in small streams, springs, and seeps in theSoutheast
and Pacific Northwest. Two factors con-tribute to specialists’
small ranges: their limited abil-ity to move between headwaters and
high diversity ofheadwater habitats. Unlike mobile animals, such
asmammals and birds, fully aquatic animals like fishand most
mollusks cannot move from one headwa-ter stream to another. As a
result, local evolution mayproduce different species in adjacent
headwater sys-tems. Moreover, environmental conditions often
dif-fer greatly between adjacent headwater streams andeven within
the course of a single stream. For exam-ple, in a spring-fed
headwater stream in westernPennsylvania, one species of caddis fly
inhabits head-
waters starting at the spring and going downstreamabout 200
yards. A different species of caddis flyinhabits the stream after
that point.
Animals may use headwater streams for all or partof their lives.
Although many fish species live exclu-sively in headwater systems,
others use headwatersonly for key parts of their life cycle. For
example,headwaters are crucial for the diversity of salmonstocks in
the Pacific Northwest because salmonspawn and rear in headwater
streams. In other partsof the country, trispot darters, brook trout
andrainbow trout spawn in small streams. Young cut-throat trout use
shelter formed by streams’ debrisdams but move onto larger portions
of a streamnetwork as they mature. Intermittent streams canoffer
special protection for young fish, because thesmall pools that
remain in such streams often lackpredators. Still other fish
species use headwaterstreams as seasonal feeding areas.
Both permanent and intermit-tent streams provide valuablehabitat
for microorganisms,plants and animals. Generally,biodiversity is
higher in perma-nent streams than in intermittent streams,
butintermittent streams often provide habitat for dif-ferent
species. Some species that occur in bothtypes of streams may be
more abundant in preda-tor-free intermittent streams. For
example,because of the lack of large predatory fish, sala-manders
and crayfish are sometimes more abun-dant in fishless intermittent
streams rather thanthose with permanent flow. In contrast, for
ani-mals such as brook trout that require steady watertemperatures
and constant water flow, perennialstreams provide better
habitat.
A water shrew (Sorex
palustris) in the water’s of
Oregon’s Mt. Hood. Photo
courtesy of RB Forbes,
Mammal Images Library
A westslope cutthroat trout
from Deep Creek, a
headwater of the Kettle
River. Cutthroat trout
spawn in headwaters
where the young trout seek
shelter amid piles of debris,
moving on to larger waters
for their adult lives. Photo
courtesy of Bill McMillan,
Washington Trout
A coho salmon migrating up
a spring-fed tributary of the
Snoqualmie River watershed
in Washington’s Puget Sound
region. Many anadromous
fish species spawn in head-
water streams that are so
small as to be omitted from
standard USGS topographi-
cal maps. Photo courtesy of
Washington Trout.
-
18
LINKAGES BETWEEN HEADWATER ANDSTREAMSIDE ECOSYSTEMS
BOOSTBIOLOGICAL DIVERSITYThe movement of plants and animals
betweenheadwater and streamside ecosystems boosts biodi-versity in
both areas. Headwater streams are tightlylinked to adjacent
riparian ecosystems, the zonesalong a stream bank. Riparian
ecosystems havehigh species diversity, particularly in arid
environ-ments where the stream provides a unique micro-climate.
Typical riparian vegetation depends uponmoist streamside soils.
Some plants must have “wetfeet,” meaning their roots have to
stretch into por-tions of soil that are saturated with water. Seeds
ofsome riparian plants, such as those of cottonwoodtrees found
along rivers in the Southwest, requireperiodic floods to germinate
and take root.
Another link between stream and land is oftenprovided by
insects, such as mayflies, that emergefrom streams and provide a
vital food resource foranimals, including birds, spiders, lizards
and bats.For example, insect-eating birds living by a prairiestream
in Kansas consume as much as 87 percentof the adult aquatic insects
that emerged from thestream each day. Such exchanges between
landand water help maintain animal populationsacross landscapes. In
many landscapes, the net-work of headwater streams is so dense that
itoffers a nearly continuous system of intercon-nected habitat for
the movement of mobilespecies that rely on streams and riparian
areas.
BIOLOGICAL DIVERSITY OF HEADWATERSYSTEMS IS THREATENED BY
HABITATDESTRUCTIONBecause of their small size and intimate
connec-tions with surrounding landscape, headwatersand their
inhabitants are easily influenced byhuman activities in watersheds
and riparianzones. Changes to riparian vegetation or hydrol-ogy,
water pollution, or the introduction ofexotic species can have
profound effects on biotaliving in headwaters.
Specialized headwater species can be particularlysensitive to
habitat destruction because of theirsmall geographic ranges,
sometimes as small as asingle headwater stream or spring. Thus,
humanactivities have driven some headwater specialists,like the
whiteline topminnow, to extinction, andimperiled many others.
Furthermore, as the nat-ural disjunction of headwater systems
isincreased by human activities such as pollution,impoundment, and
destruction of riparian vege-tation, more populations of headwater
specialistsmay be extirpated.
Many headwater species, including fish, snails,crayfish, insects
and salamanders, are now indanger of extinction as a result of
humanactions. A few dozen headwater species arealready listed under
the U.S. Endangered SpeciesAct; hundreds of others are rare enough
to beconsidered for listing. Given the diversity andsensitivity of
headwater biota, it seems likely thatcontinued degradation of
headwater habitatswill put more species at risk of extinction.
Canelo Hills ladies' tresses
[Sprianthes delitescens] in a
southwestern freshwater
marsh known as a cienega.
The cienegas of Arizona and
New Mexico and Mexico, are
the exclusive habitat for this
member of the orchid family.
Photo courtesy of Jim
Rorabaugh, USFWS
The Cleistes, a member of the
orchid family, is found in
pocosin wetlands of North
Carolina. Photo courtesy of
Vince Bellis
-
19
WETLANDS MAKE KEY CONTRIBUTIONS TOBIOLOGICAL DIVERSITYThe
presence of wetlands adds another aspect ofhabitat diversity to
headwater systems and there-fore increases the variety of species a
headwatersystem may support. Most headwater wetlandsare depressions
in the ground that hold waterpermanently or seasonally. Wetlands
providecritical habitat for a variety of plants and ani-mals.
Scientists usually distinguish betweenephemeral and perennial
wetlands.
BIODIVERSITY IN EPHEMERAL WETLANDSSome species of plants and
animals prefer orrequire ephemeral wetlands. Certain zooplank-ton,
amphibians, and aquatic plants need thewet phase of an ephemeral
wetland to completeall or part of their life cycles. Other species
thatrely on ephemeral wetlands wait out the aquaticphase,
flourishing only when pools shrink or dis-appear. For example,
although adult spottedsalamanders are generally terrestrial, during
thespringtime they trek to vernal pools to breed andreproduce.
So-called amphibious plants, includ-
ing button celery, meadowfoam, wooly marblesand many others do
the opposite; although theylive in water, they cannot reproduce
until waterlevels drop. Some plants and crustaceans moststrongly
identified with ephemeral wetlandsworldwide, including quillworts,
fairy shrimp,and tadpole shrimp, are ancient groups thatprobably
originated at least 140 million yearsago. The disappearance of
ephemeral wetlandswould mean the loss of these highly
specializedand ancient groups of plants and animals.
One type of ephemeral wetland found in bothCalifornia and the
Northeast is known as a ver-nal pool because it generally fills
with water inthe spring. In California, blooming flowers ringthe
edges and fill depressions of such pools. Ofthe 450 species,
subspecies, or varieties ofplants found in California’s vernal
pools, 44 arevernal pool specialists. Several such plants
arealready on the Endangered Species list. IfCalifornia’s vernal
pool habitats were com-pletely destroyed, at least 44 species would
dis-appear. Although vernal pool animals are lesswell known, there
appear to be at least as many
Pitcher plants, such as this
white top (Sarracenia leuco-
phylla), pictured top left; and
sundews, such as this
Drosera brevifolia, pictured
bottom right; are among the
carnivorous plants found in
the Carolina Bay wetlands of
the Southeastern U.S. Photo
courtesy of David Scott/SREL
-
20
specialized animals as plants. New species ofspecialists such as
fairy shrimp and clam shrimpcontinue to be discovered.
Other ephemeral wetlands also make significantcontributions to
biodiversity. A study of wetlandsin the Southeast including
cypress-gum swamps,cypress savannas, and grass-sedge marshes,
foundthat plants from one wetland are often very dif-ferent from
those in others nearby. Such differ-ences in nearby habitats
increase overallbiodiversity in a region. In some cases,
differencesin periods of wetting and drying appear to beimportant
for the persistence of many species.Different wetting and drying
patterns explainsome differences between Gromme Marsh and
Stedman Marsh, two prairie pothole wetlands inWisconsin.
Although the two marshes are onlyabout 450 yards apart, they have
different speciesof dragonflies; also, Stedman Marsh has
dam-selflies and caddis flies that Gromme Marsh lacks.
Amphibians are key parts of the food web insmall wetlands. Some
wetlands are hot spots foramphibian biodiversity; twenty-seven
amphib-ian species, one of the highest numbers ofamphibian species
known from such a smallarea, inhabited a 1.2-acre ephemeral wetland
inSouth Carolina. Other small wetlands in theregion have been found
to have similar numbersof amphibian species, demonstrating how
smallwetlands are especially important for maintain-ing the
regional biodiversity of amphibians.Larger, more permanent wetlands
may be lessdiverse because they may also be home to preda-tors-such
as crayfish and dragonfly larvae-thateat amphibian larvae.
BIODIVERSITY IN FENS (A TYPE OF PEREN-NIAL WETLAND)Plant
biodiversity peaks in fens, unique peren-nial wetlands that occur
where groundwaterflows to the surface. Fens also provide cleanwater
that supports downstream ecosystems;outflows from such wetlands are
critical to theformation of the cold, low-nutrient streams thatare
ideal for trout. Although fens are rarely inun-dated, water seeps
continuously into root zones.
Similar to other wetlands, the small land areacovered by fens
belies the high biodiversityfound within them. For example, in
northeast-ern Iowa, fens contain 18 percent of the state’splant
species but cover only 0.01 percent of theland surface. Fens are
probably the wetlandswith the greatest numbers of plant
species.Because groundwater that comes to the surface istypically
low in available nutrients, fen plants areoften dwarfed and the
total mass of vegetation istypically low. As a result, no one
species canbecome dominant and exclude other species.
In the Upper Midwest, more than 1,169 speciesof plants have been
identified in fens, with morethan half needing wet conditions. Fens
also have
Although spotted salaman-
ders are generally terrestrial
animals, they only breed and
reproduce in vernal pools.
Photo courtesy of Vernal
Pool Association
A female fairy shrimp from
the Ipswich River Basin in
Massachusetts. Fairy shrimp
spend their entire life cycles
in vernal pools. Photo cour-
tesy of Vernal Pool
Association
-
21
a high proportion of plant species known tooccur primarily in
pristine sites. Often, suchspecies are listed as rare, threatened
or endan-gered. Of 320 vascular plant species foundwithin fens in
northeastern Iowa, 44 percent areconsidered rare. Fens themselves
are imperiled:160 fens that one researcher sampled in north-eastern
Iowa were all that remained from 2,333historic fens.
Because diversity in fens stems from low nutrientavailability,
overfertilization can harm fens and,in turn, downstream ecosystems.
Examining onefen in New York, researchers found the lowestdiversity
of plants where nitrogen and phospho-rus inflows were greatest.
Both nutrients camefrom agricultural activities: phosphorus
wasentering the fen primarily through surface waterflows, while the
nitrogen-containing compoundnitrate was flowing with the
groundwater. Thus,a loss of plant diversity in fens is a clear
indica-tion they are receiving excess nutrients, such ascan occur
when fertilizer runs off a field or urbanlawn or water carries
animal waste from farm-
yards. Allowing excess nutrients to enter fenscan also damage
downstream trout streamsbecause trout prefer cold, low-nutrient
streams.Therefore, the low-nutrient conditions of fensrequire
protection from nutrient contamination.
A wood frog (Rana sylvatica)
in an autumnal vernal pool
in central Pennsylvania.
Photo courtesy of
Gene Wingert
Fens are unique perennial
wetlands that occur where
groundwater flows to the
surface. Plant biodiversity
peaks in fens: Among the
320 vascular plant species
found in northeastern Iowa
fens, 44% are considered
rare. However, fens them-
selves are imperiled. Pictured
is a fen wetland in Illinois.
Photo courtesy of Steve
Byers, Bluff Spring Fen
Nature Preserve
-
22
Conclusion
Headwater streams and wetlands aboundon the American landscape,
providingkey linkages between stream networks and sur-rounding
land. Although often unnamed,unrecorded, and underappreciated,
small headwa-ter streams and wetlands-including those that aredry
for parts of the year-are an integral part of ournation’s river
networks. Small wetlands, eventhose without visible surface
connections, arejoined to stream systems by ground-water,
subsurface flows of water, andperiodic surface flows.
Currentdatabases and maps do not ade-quately reflect the extent of
headwa-ter streams and associated wetlands.The resulting
underestimate of theoccurrence of such ecosystems ham-pers our
ability to measure the keyroles headwater systems play
inmaintaining quality of surfacewaters and diversity of life.
Essential ecosystem services providedby headwater systems
include attenu-ating floods, maintaining water sup-plies,
preventing siltation of downstream streams
and rivers, maintaining water quality, and support-ing
biodiversity. These small ecosystems also providea steady supply of
food resources to downstreamecosystems by recycling organic
matter.
Small streams and wetlands provide a rich diversityof habitats
that supports unique, diverse, andincreasingly endangered plants
and animals.Headwater systems, used by many animal species
atdifferent stages in their life history, provide shelter,
food, protection from predators,spawning sites and nursery
areas, andtravel corridors between terrestrialand aquatic
habitats.
Since the 1970s, the federal CleanWater Act has played a key
role inprotecting streams and wetlandsfrom destruction and
pollution. Wehave made progress toward cleanerwater, in part
because the law hashistorically recognized the need toprotect all
waters of the UnitedStates. The health of downstreamwaters depends
on continuing pro-
tection for even seemingly isolated wetlands andsmall streams
that flow only part of the year.
These small streams and wetlands are beingdegraded and even
eliminated by ongoinghuman activities. Among the earliest and
mostvisible indicators of degradation is the loss ofplant diversity
in headwater wetlands. The phys-ical, chemical, and biotic
integrity of our nation’swaters is sustained by services provided
by wet-lands and headwater streams.
Today’s scientists understand the importance ofsmall streams and
wetlands even better than theydid when Congress passed the Clean
Water Act.If we are to continue to make progress towardclean water
goals, we must continue to protectthese small but crucial waters.
The goal of pro-tecting water quality, plant and animal
habitat,navigable waterways, and other downstreamresources is not
achievable without careful pro-tection of headwater stream
systems.
Photo courtesy of
Raymond Eubanks.
“THE PHYSICAL,
CHEMICAL, AND
BIOTIC INTEGRITY OF
OUR NATION’S
WATERS IS SUSTAINED
BY SERVICES PRO-
VIDED BY WETLANDS
AND HEADWATER
STREAMS.”
-
23
R E F E R E N C E S :
The authors used more than 235 scientific publications
inpreparing this paper. This brief list is intended to provide
thereader with an introduction to the relevant scientific
literature.
Alexander, D.R., and H.R. MacCrimmon. 1974. Production
andmovement of juvenile rainbow trout (Salmo gairdneri) in a
headwa-ter of Bothwell’s Creek, Georgian Bay, Canada. Journal of
theFisheries Research Board of Canada 31: 117-121.
Amon, J.P., C.A. Thompson, Q.J. Carpenter, and J. Miner.
2002.Temperate zone fens of the glaciated Midwestern USA.
Wetlands22(2): 301-317.
Arnold, C. L., P.J. Boison, and P.C. Patton. 1982. Sawmill
Brook- An example of rapid geomorphic change related to
urbanization.Journal of Geology 90:155-166.
Bedford, B.L., D.J. Leopold, and J.P Gibbs. 2001. Wetland
ecosys-tems. P. 781-804. In Encyclopedia of Biodiversity, Volume
5.Academic Press, San Diego, CA, USA.
Benz, G.W. and D.E. Collins (editors). 1997. Aquatic fauna
inperil: the southeastern perspective. Southeast Aquatic
ResearchInstitute Special Publication 1. Lenz Design and
Communications,Decatur, GA.
Beven, K. and M. J. Kirkby (ed.) 1993. Channel NetworkHydrology.
New York: John Wiley and Sons.
Curry, R.A., C. Brady, D.L.G. Noakes, and R.G. Danzmann.
1997.Use of small streams by young brook trout spawned in a
lake.Transactions of the American Fisheries Society 126: 77-83.
Dieterich, M. Anderson N. H. 2000. The invertebrate fauna
ofsummer-dry streams in western Oregon. Archive fur
Hydrobiologie147:273-295.
Erman, D.C., and V.M. Hawthorne. 1976. The
quantitativeimportance of an intermittent stream in the spawning of
rainbowtrout. Transactions of the American Fisheries Society 105:
675-681.
Gibbs, J.P. 1993. Importance of small wetlands for the
persistenceof local populations of wetland-associated animals.
Wetlands.13:25-31.
Gomi, T., R. C. Sidle, and J. S. Richardson. 2002.
Understandingprocesses and downstream linkages of headwaters
systems.BioScience 52:905-916.
Hansen, W. F. 2001. Identifying stream types and
managementimplications. Forest Ecology and Management
143:39-46.
Knox, J. C. 1977. Human impacts on Wisconsin stream
channels.Annals of the Association of American Geographers
67:323-342.
Labbe, T.R., and K.D. Fausch. 2000. Dynamics of
intermittentstream habitat regulate persistence of a threatened
fish at multiplescales. Ecological Applications 10: 1774-1791.
Leopold, L. B. 1994. A View of the River. Cambridge,
Mass:Harvard University Press.
Lowrance, R.R. and others. 1997. Water quality functions of
ripar-ian forest buffers in Chesapeake Bay watersheds.
EnvironmentalManagement 21: 687 - 712.
Meyer, J. L. and J. B. Wallace. 2001. Lost linkages and lotic
ecology:rediscovering small streams. Pages 295-317 in M.C. Press,
N.J.Huntly, and S. Levin, editors. Ecology: achievement and
challenge.Blackwell Science.
Ohio Environmental Protection Agency. 2002a. Clean riversspring
from their source: the importance and management of head-waters
streams. Columbus, Ohio: State of Ohio EnvironmentalProtection
Agency, Division of Surface Water.
Roni, P. 2002. Habitat use by fishes and Pacific giant
salamandersin small western Oregon and Washington streams.
Transactions ofthe American Fisheries Society 131: 743-761.
Russell, K.R., D.C. Guynn, and H.G. Hanlin. 2002. The
impor-tance of small isolated wetlands for herpetofaunal diversity
in man-aged, young growth forests in the Coastal Plain of South
Carolina.Forest Ecology and Management 163(1-3): 43-59.
Semlitsch, R.D., and J.R. Bodie. 1998. Are small, isolated
wetlandsexpendable? Conservation Biology 12:1129-1133.
Tiner, R.W., H.C. Bergquist, G.P. DeAlessio, and M.J. Starr.
2002.Geographically isolated wetlands: A preliminary assessment of
theircharacteristics and status in selected areas of the United
States.USDA Fish and Wildlife Service, Northeast Region, Hadley,
MA.http://wetlands.fws.gov/Pubs_Reports/isolated/report.htm
Waterhouse, F.L., A.S. Harestad, and P.K. Ott. 2002. Use of
smallstreams and forest gaps for breeding habitats by winter wrens
incoastal British Columbia. Northwest Science 76: 335-346.
Wipfli, M. S., and D. P. Gregovich. 2002. Export of
invertebratesand detritus from fishless headwater streams in
southeastern Alaska:implications for downstream salmonid
production. FreshwaterBiology 47:957-969.
Zedler, J.B. 2003. Wetlands at your service: reducing impacts
ofagriculture at the watershed scale. Frontiers in Ecology 1 (2):
65-72.
FOR ADDITIONAL INFORMATION ON TOPICS DISCUSSED INTHIS REPORT,
READERS MAY CONSULT THE
FOLLOWINGWEBSITES:http://www.nwrc.usgs.gov/
http://www.cwp.org/pubs_download.htm
http://www.epa.gov/OWOW/NPS/urbanize/report.html