Streams

Streams•To geologists, a stream is any flowing body of water in a channel—brook, creek, wash, wadi, slough, stream, river…

•Streams flow in valleys they have carved. Small streams have small valleys, big streams have big valleys.

•Streams form hierarchical networks of connected streams, where one stream feeds into another that eventually flows into the ocean or other closed depression.

•Streams are fed by 1) other streams; 2) groundwater discharge into the channel; and 3) runoff from slopes.

• Most landscapes (spatially and temporally) are dominated by stream flow.

–Present Upper Midwest is unusual because glaciation has produced a non-stream landscape.

–Areas like Missouri and Texas are more typical.

•When geologists speak of stream processes, they include not only the channel processes, but also the mass wasting and sheetflow and erosion associated with channels.

v 0054 of 'Streams' 10Streams.pptby Greg Pouch at 2013-09-30 13:14:58 LastSavedBeforeThis 2011-09-25 14:14:13

Streams3 Hydrologic Cycle4 Anatomy of a stream5 Vocabulary6 Streamflow>Overview

7 Streamflow>Variation8 Streamflow>Meanders

9 Erosion and Transport of Particles by Water10 Stream Processes>Overview

11 Stream Processes>Erosion12 Stream Processes>Deposition

13 Stream Processes>Deposition>Terminal14 Stream Processes>Deposition>Channel15 Stream Processes>Deposition>Terraces

16 Stream Processes>Erosion Hills, Slopes, Valleys and Stages of Landform Development17 Stream Processes>Erosion>Landform Development>Youth18 Stream Processes>Erosion>Landform Development>Mature19 Stream Processes>Erosion>Landform Development>Old Age

20 Drainage patterns21 Channel Types

Hydrologic Cycle• Major components

–Main location of evaporation is oceans. Main location of precipitation is oceans.

–Because land is dry, there is not as much opportunity for evaporation, so land experiences net precipitation

• What happens to rain that falls on land?–Evaporation directly to atmosphere–Transpiration by plants–Infiltration into soil then groundwater then into

a stream• Infiltration ultimately becomes transpiration

or runoff (or [rare] groundwater discharge into an ocean or [rare] evaporation)

–Runoff to channels• Rainfall onto land either evapotranspires or runs

into stream channels

Anatomy of a stream• The region that drains into a particular stream is the drainage basin.

• The region that usually or most often has water in it is the channel.

• Often, there is a broad flat plain adjacent to the channel that gets inundated during floods and is called a floodplain.

• Outward from the floodplain are the valley wall and uplands

• Frequently, there are channel-parallel ridges next to the channel called levees, that result from deposition of suspended load during floods.

• The sediment, if present, underlying the channel and floodplain is called valley fill.

Valley Wall

LeveeChannel

Floodplain

LeveeFloodplai

n

Floodplain

Floodplain

Vocabulary• Gradient is the slope of the channel, measured along the

channel. Decreases downstream.• Width of channel is measured perpendicular to direction

of flow, and varies with time. Increases downstream.• Thalweg is the line down a channel connecting the deepest

point at each cross section. This is where fastest flow occurs.

• Depth of channel varies tremendously, with time, along channel, and across channel. Depth usually increases downstream. Usually, people mean the deepest depth at a given location at a given time. Stage refers to depth of water in channel measured at a gage station.

• Discharge is the amount of water flowing past a point, usually measured in cubic feet per second or cubic meters per second.

• Longitudinal profile a cross-section along the direction of flow. They have a characteristic shape that looks like an exponential decay. They don’t change rapidly.

• Transverse or latitudinal profile goes across the channel. They have a variety of shapes and change rapidly.

• Graded streams A graded stream, in net and on average, neither erodes nor deposits Most streams are graded for most of their length, but streams almost always erode into their headlands and deposit at their mouth.

Streamflow>Overview• Streamflow is expressed as volume per unit time, usually cubic

meters per second, cubic feet per second, sometimes cubic kilometers per second, or acre-feet per second or day.

• Streamflow varies tremendously with time. Short term controls include rainfall, snowmelt, and evaporation conditions. Long term controls include landuse, soil, groundwater state, and rock type.

• Streamflow is measured at gauging stations, where the stream is routed into an artificial channel of known cross section, often a V, or the dimensions of the channel have been measured (and need to be re-measured as channels shift and sediments accumulate...) The height in the channel is recorded continuously, and converted into stream flow.

• A graph of flow vs. time is called a hydrograph. The shape of a hydrograph depends on rainfall, landuse, geology, and soils.

• Example Hydrograph: Sangamon River at Riverton IL http://waterdata.usgs.gov/il/nwis/uv?cb_all_00060_00065=on&format=gif_stats&period=31&site_no=05576500

Streamflow>Variation• At a given location, as discharge increases, the channel deepens, widens, and flows faster.

• As you go downstream in a humid region, discharge increases, so

– the channel deepens and widens, and

– velocity usually slows down, but this varies from stream to stream, depending on how much the channel widens

• In an arid region, water lost to evaporation might exceed water gained from tributaries and groundwater, resulting in a decrease in discharge as you go downstream.

• Velocity in a stream depends on discharge (largely controlled by recent weather), channel shape, and channel roughness. The velocity of flow in a stream, especially a small one, varies tremendously with time, because the short-term discharge varies with the weather. For small streams, variations in discharge of a factor of 1,000 are very common. Larger streams tend to be more stable.

Streamflow>Meanders• Water tends to move downstream in slugs

that extend all the way across a channel. • When the channel bends, water on the

outside of the bend (the cut-bank) flows faster and water on the inside of the bend (the point) flows slower. This distribution of velocity results in erosion occurring on the outside of the bend (cut) and deposition occurring on the inside of the bend.

• Because of this positive feedback loop, meanders get bigger (meander more), until finally the meander gets cut off, usually in a flood.

Erosion and Transport of Particles by Water• Velocity and Competence and Capacity Discuss Figure 10.7 (velocity versus competence curve). In short, higher velocity is

required to move larger particles.• Modes of transport

–Bedload• Sliding• Rolling• Traction• Saltation

–Suspended–Dissolved

1cm/sec=3600cm/hr=36m/hr

100cm/sec=3600m/hr=3.6km/hr=~2 miles per hour

Stream Processes>OverviewStreams do a lot of things. These processes are interwoven, and often occur simultaneously, sometimes in

seemingly contradictory ways (For example, at a meander, a stream is eroding one side of its channel and depositing on the other.) Any, all, or none of these processes may be happening at a location, and the result of stream processes is largely the combination of them.

• Move water is the essence of a stream.

Because streams move water, they:

• Transport sediments

• Deposit sediments when they slow down

• Erode Headward: extending the drainage into un-drained uplands, if there are any. Gully formation.

• Downcut: eroding down into stream bed

• Erode Laterally their banks, valley fill, and valley walls. This also leads to oversteepened slopes that are susceptible to mass wasting. Combined, lateral erosion and mass wasting produce slope retreat.

Because stream flow varies dramatically, at a single location, deposition and erosion may alternate on a daily or yearly or ?centurily? … basis.

Stream Processes>Erosion

Streams erode by a combination of direct stream processes, like downcutting and lateral erosion, and indirect processes, like mass-wasting accompanied by transportation.

Stream Processes>Deposition

• Any place or time the stream slows down, it loses capacity and competence, and deposits sediments. Stream sediments are often re-eroded by the stream itself, but may get preserved or are visible in their initial state.

• Channel sediments may be subject to continuous re-working and result in very well-sorted sediments.

• In cut-off meanders, deltas, and alluvial fans, sediments are not re-worked and they are less well-sorted.

• “Deposits” can also form by removing other material. For example, there are channel lag "deposits" consisting of material too coarse for the stream to move and often contain fossils like dinosaur bones and petrified wood.

Stream Processes>Deposition>TerminalWhere a stream loses gradient suddenly, such as where it enters a large body of standing water or flows out

onto a valley floor, it slows down and rapidly drops a lot of sediments. These pile up and further reduce gradient, until an easier (steeper/shorter) path is found and the stream takes that path. The result on land is alluvial fans, which are surprisingly circular cones. In water, there are more processes (waves and tides especially) and more complicated shapes arise for deltas, such as bird’s foot deltas or delta-shaped deltas.

• Deltas (deposits left when a stream stops where it hits a standing body of water, like an ocean or a lake)• Alluvial fans (deposits left when a stream stops where it hits a great decrease in slope, going from

mountains to valley)

Fig. 10.18

Stream Processes>Deposition>Channel• Meanders

–Cutbank On the outside of a meander, the current is fast and erosion occurs.

–Pointbars On the inside, the current is slow and deposition occurs. These start as coarse deposits and, as the location gets shallower, the sediments get finer, so you get a fining upward sequence.

• Levees When a river overflows its banks, the water loses velocity and coarse, suspended load (mainly silt, some sand) gets deposited in linear ridges parallel to the channel.

• Floodplains When a river overflows its channel, there is usually an extensive, flat lowland area (the floodplain) that gets flooded. The water here flows very slowly, so only clay and some silt can get transported here. Very fertile soils due to the sporadic influx of fresh soil materials.

Stream Processes>Deposition>Terraces• Terraces These are floodplain or channel deposits that are now above the channel level. They are often

very well drained, because the water table is about level with the river and there are permeable materials (usually old channel-bed deposits) that conduct water very well.

Stream Processes>ErosionHills, Slopes, Valleys and Stages of Landform DevelopmentThe channel is not the only stream-sculpted part of the landscape. The slopes that

feed into it (slopes, and hills) are also shaped by streams and surface runoff. Most landscapes are dominated by streams.

Stages of stream development Landscape development as a series of steps ("cycle")Diagrams at right from http://www.uwgb.edu/dutchs/EarthSC202Notes/erosion.htm NEXT THREE SLIDES HAVE DETAILS AND PHOTOS• Youth In youth (initial state) an area has been uplifted and you have a flat upland.

There are few streams, they usually have straight, narrow V-shaped valleys with steep gradients.

– Upland swamps are common. Part of the upland area does not drain into streams. Headward erosion, down cutting and slope retreat are all active. No floodplains. Streams are, in net, eroding. Deposits are few.

– Good for mosquitoes. Soils may be very well-drained if the underlying materials are permeable.

– Common in Midwest.• Mature Erosional processes are slowing down, headward erosion has more or less

stopped. An integrated drainage system has formed, and the entire area drains into streams. (The whole area is in slope.)

– Streams are broad V-shaped or \_/,.all major streams and some minor ones meander, the meanders occupy the entire floodplain, and gradients are medium. Floodplains are narrow. Both depositional and erosional features are found, with erosion slightly outpacing deposition.

– Bad for mosquitoes. Soils are well-drained. – Common in “shallow” South (Missouri, Kentucky, Tennessee… )

• Old Age Erosion has largely ceased and deposition dominates. – Valleys are very broad, but meanders occupy a small part of the floodplain,

gradients are low to very low. Levees are common, and the stream may be higher than the floodplain, resulting in backswamps and yazoo streams. Lowland swamps are common, as are oxbow lakes and cutoff meanders.

– Good for mosquitoes. Lots of poorly drained soils, flooding common.– Common in Deep South (Louisiana , Mississippi…)

Stream Processes>Erosion>Landform Development>YouthDiagrams at right from http://www.uwgb.edu/dutchs/EarthSC202Notes/erosion.htm

• Youth In youth (initial state) an area has been uplifted and you have a flat upland. –There are few streams.–Streams usually have straight, narrow V-shaped valleys with steep gradients.–Upland swamps are common. Part of the upland area does not drain into streams. –Headward erosion, down cutting and slope retreat are all active. –No floodplains have formed.–Streams are, in net, eroding. Deposits are few. –Waterfalls are present. –Rocky streams are common.–Good for mosquitoes (upland swamps). –Soils may be very well-drained if the underlying materials are permeable. –Common in Midwest.

Fig. 10.37d

0094

Stream Processes>Erosion>Landform Development>Mature• Mature

–Erosional processes are slowing down.

–Headward erosion has more or less stopped (no undrained uplands left).

–An integrated drainage system has formed, and the entire area drains into streams. (The whole area is in slope.)

–Streams are broad V-shaped or \_/.

–All major streams and some minor ones meander, the meanders occupy the entire floodplain, and gradients are medium.

–Floodplains are narrow.

–Both depositional and erosional features are found, with erosion slightly outpacing deposition.

–Bad for mosquitoes. Soils are well-drained.

–Common in “shallow” South (Missouri, Kentucky, Tennessee… )

• Old Age Erosion has largely ceased and deposition dominates.

–Valleys are very broad, but meanders occupy a small part of the floodplain.

–Gradients are low to very low.

–Levees are common, and the stream may be higher than the floodplain, resulting in backswamps and yazoo streams.

–Lowland swamps are common, as are oxbow lakes and cutoff meanders.

–Good for mosquitoes. Lots of poorly drained soils, flooding common.

–Common in Deep South (Louisiana, Mississippi…)

Stream Processes>Erosion>Landform Development>Old Age

Drainage patterns• Drainage patterns (shape of connected hierarchy of streams) reflect the underlying geology.

–Dendritic (looks like a veins in a maple leaf, or branches in a tree) results from uniform material. If there is a regional tilt, the drainage network looks more like palm leaves.

–Radial indicates flow out of a central, circular upland, like a volcano.

–Rectangular indicates a jointed or faulted pattern, with the joints or faults intersecting at right angles, or a rectangular distribution of rocks type as found in an area of downdropped fault blocks. A variant is angular drainage, where the intersections are oblique. In addition to meeting at right angles, streams have right angle bends.

–Trellis system develops in areas of dipping (tilted) sediments where alternating beds are resistant or susceptible to erosion. The main valleys are over susceptible-to-erosion rocks, with short/side valleys draining the ridges formed over resistant-to-erosion rocks.

–Centripetal (streams flow inward, rare) indicates a central, circular lowland like a sinkhole.

Channel Types• Rills

• Gully

• Straight Usually V-shaped, typically steep gradient, often symmetric. Common in uplands and early stages of stream development.

• Meandering Usually broadly U-shaped, asymmetric. Common in lowlands and delta regions, late stages of stream development. Mature (meanders occupy whole floodplain) and old age (channel occupies small part of floodplain)

• Braided multiple interconnecting (anastomosing) channels. Common in arid climates, sands, and channels carrying much of its load in coarse materials.

• Wadi, Arroyo

Streams• Most landscapes (spatially and temporally) are

dominated by stream flow. Present Upper Midwest is unusual because glaciation has produced a non-stream landscape. Areas like Missouri and Texas are more typical.

• Stream-dominated landscapes vary greatly.

• Stream processes often occur over/with other processes. (e.g., McLean County, IL was glaciated and is showing youthful to mature stream landscapes)

• Streams and mass wasting work together to create stream eroded landscapes with integrated slopes leading into streams.

• The basic stream processes occur in all stream landscapes, but the particular processes (downcutting, headward erosion, meandering through valley fill…) depend on the materials available and how far the processes have gone (age)

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Hydrologic Cycle, Big Version