Geomorphology of streams 1: stream classifications Dr. John Richardson FRST 386 courses.forestry.ubc.ca/frst386/
Geomorphology of streams 1: stream classifications
Dr. John RichardsonFRST 386
courses.forestry.ubc.ca/frst386/
Terms to define parts of a stream network
Classifications of streams
How watersheds are formed
Processes that lead to large-scale features
Outline
Geomorphology (or “fluvial geomorphology”)
Morphology of stream networks – physical arrangements of the pieces, but especially the study of the processes creating those arrangements
Scales from whole watersheds to the exact location of a bacterium on a piece of rock
We’ll include the stream channel, and the surroundings that the water works it way through (watershed, floodplain), including the closest part, referred to as the riparian area.
Catchment
Watershed
Basin (or drainage basin)
From hydrology we would call this the contributing area, i.e., the area generating flow to the channel
In North America we often use “watershed”, but in other places, and more correctly, watershed means the divide between catchments. These three terms are often used interchangeably, but the preferred term is catchment.
Channel
Hillslope (or “upslope”)
Floodplain
Terraces (of different ages)
Parts of a catchment
Channel – characterised by mineral bed material, evidence of fluvial processes
Active channel – reworked on at least annual basisWetted channel – water in the channel at time of observationBankfull channel – margin where annual, or active, channel gives way to the true channel margin or bank
Floodplain – evidence of occasional overtopping of banks
Terraces – built up, former floodplains, now vegetated (usually) and above the floodplain level
Hillslope – the part of the contributing area beyond the floodplain (doesn’t have to have much slope)
Channel
Hillslope (or “upslope”)
Floodplain
Terraces (of different ages)
Parts of a catchment
Hierarchical classification of stream habitats showing approximate linear spatial scales
Stream system - catchments
Frissell CA, Liss WJ, Warren CE & Hurley MD. 1986. A hierarchical framework for stream habitat classification: viewing streams in a watershed context. Environmental Management 10:199–214.
Classifications
We use streams to mean all flowing waters, incl. brooks, becks, branch, creeks, streams, rivers, etc.
In Europe, in particular, there is a system of classification not often used in North America
Krenon or Kryalspring source
RhithronSmall streams
PotamonLarge streams
Stream order
Stream link number (cumulative number of 1st order streams)
Some measure of average annual discharge or unit discharge
flow
Stream orders Proposed originally by Strahler (1954)
Based on a map scale of 1:50,000
1
23
1
1
1
2
2
11
1
3
1
2
3
4
Link size = 12
Stream orders
Depends on map scale
Depends on landscape
Small streams very easily missed, due to canopy closure
Several estimates that ~80% of small, permanently flowing streams are missed (Meyer & Wallace 2000)
Corollary: small streams very heavily influenced by their surrounding forests
Vannote RL, Minshall GW, Cummins KW, Sedell JR & Cushing CE. 1980. The River Continuum Concept. Canadian Journal of Fisheries and Aquatic Sciences 37: 130-137.
Riparian class
Average channel width (m)
Reserve zone width (m)
Management zone width (m)
Total RMA width (m)
S1 large rivers
≥ 100 0 100 100
S1 (except large rivers
> 20 50 20 70
S2 > 5, ≤ 20 30 20 50
S3 1.5 to 5 20 20 40
S4 < 1.5 0 30 30
S5 > 3 0 30 30
S6 ≤ 3 0 20 20
BC’s Riparian Management Area Guidelines (1995)
BC Ministry of Forests (1995) Riparian Management Area Guidebook
Shaded: Fish stream or community watershed
River Continuum Concept
Vannote RL, Minshall GW, Cummins KW, Sedell JR and Cushing CE. 1980. The River Continuum Concept. Canadian Journal of Fisheries and Aquatic Sciences 37: 130-137.
Gradual transitions in many properties of streams, not easily captured by strict classification
Properties of streams depend on what goes on upstream
A somewhat idealised concept, but useful for many generalisations about streams
Area or length0.1 1 10 100
Mag
nitu
de
Stored sediment
Stream flow
Size of bed material (grain size)
Channel gradient
As stream size increases
Stream flow (Q)
Amount of stored sediment
Channel gradient
Grain size
Tractive force Tg (taug) in N/m2 (also referred to as shear stress)
Τg = ρ · g · R · S
where,
ρ = density of water (1 kg / L)
g = gravitational acceleration (9.81 m/s)
R = hydraulic radius (m) – ratio of cross-sectional area to wetted perimeter
S = slope of the energy line
Τg = ρ · g · R · S
This can be approximated as Tg ≈ D · S
i.e., depth · slope
Tractive force Tg (taug)
Most forces act on stream channel at high flows –forces can be hard to imagine
This is the primary reason we get concerned about peak flows when people discuss forest hydrology
Τg = ρ · g · R · S
This can be approximated as Tg ≈ D · S
i.e., depth · slope
Tractive force
Large, rare events (e.g. 1 in 10 year flood) can impose large changes on a channel
Some features of stream channels may last centuries from rare events, with smaller changes superimposed upon those larger-scale features
Hierarchical classification of stream habitats showing approximate linear spatial scales
Stream reaches and segments
Frissell CA, Liss WJ, Warren CE & Hurley MD. 1986. A hierarchical framework for stream habitat classification: viewing streams in a watershed context. Environmental Management 10:199–214.
Colluvial Alluvial Bedrock
colluvial braided regime pool/riffle plane-bed step-pool cascade bedrock
Transport Limited Supply Limited
Stream beds made of mineral substrates (organic materials decompose or are washed away)
A stream channel is defined as showing evidence of “fluvial processes” (fluvial = running water) such as bare sediments (e.g. no organic soils) – this is how channels were defined for purposes of the Forest Practices Code (BC 1995)
(from: Montgomery & Buffington 1993)
Alluvial channel – sediment transport and reworking
Note: energy of water at high flow rearranges the channel and its banks
Alluvial channel - Meandering reach
Bedrock dominated
Colluvial – stream power not sufficient to move dominant pieces of rock –these pieces are either in place (e.g. from glacial tills) or fall in from hillslope
Colluvial Alluvial Bedrock
colluvial braided regime pool/riffle plane-bed step-pool cascade bedrock
Transport Limited Supply Limited
These are reach types (may also be applied to stream segments)
Sources of materials to streams
A lot of material forming stream channels comes by way of mass wasting, i.e., slope failures on hillsides delivering rock, soil and wood
Mass wasting occurs naturally, but the rate is often greatly increased by land-use activities such as roads and forest harvesting
In the case of “colluvial” streams, the stream bed works down through existing materials
Channelised debris flows from upstream bring rocks and wood to downstream reaches
Debris flows also occur naturally, but the rate of them tends to increase dramatically in catchments with forest harvesting
Summary
Parts of a catchment (watershed) – channel, floodplain, terrace, hillslope
Classifications – order, for management purposes, etc.
River Continuum Concept as a general model of stream function
Tractive force – process that leads to formation of the morphology (geomorphology) of stream channels
Geomorphology of streams 2: stream channels, channel units, and hydraulic habitat
Dr. John Richardson
FRST 386
courses.forestry.ubc.ca/frst386/
Reaches, channel units, hydraulic habitat (microhabitats)
Substrate types, including large wood in streams
Describing fine-scale habitats
Outline
Hierarchical classification of stream habitats showing approximate linear spatial scales
Reach types and channel unitsGeomorphology
Frissell CA, Liss WJ, Warren CE & Hurley MD. 1986. A hierarchical framework for stream habitat classification: viewing streams in a watershed context. Environmental Management 10:199–214.
Braided, alluvial reach
Squamish River, BC
Twentieth century disturbance maps for two study reaches along the Queets River in the Olympic Mountains. The 16 dates represented are the years for which accurate spatial data were available.
Naiman RJ et al. 2010. A process-based view of floodplain forest patterns in coastal river valleys of the Pacific Northwest. Ecosystems 13:1-31.
Riffle-pool reach
Step-pool reach
Cascade (bedrock dominated)
Disturbance regimes - channelised debris flows
Infrequent for a given channelHigh magnitude effect
Channelised stream reach
Micro-habitat or hydraulic habitats
“Micro-habitat” depends on scale of organism – fish may find micro-habitat in a channel unit, whereas an invertebrate may select a very specific part of a single rock
Frissell CA, Liss WJ, Warren CE & Hurley MD. 1986. A hierarchical framework for stream habitat classification: viewing streams in a watershed context. Environmental Management 10:199–214.
Two primary kinds of “substrate” (bottom material)
Mineral – rocks, inorganic particles
Wood – large wood (>10 cm diam.), small wood (<10 cm diam.)
(e.g. trees or branches falling into streams)
Large wood in streams
Buffington JM, Lisle TE, Woodsmith RD & Hilton S. 2002. Controls on the size and occurrence of pools in coarse-grained forest rivers. River Research and Applications 18:507–531.
Functions of wood
• Creates steps that cause plunge pools
• Backs up finer sediments, forming locally gentle slopes (may include spawning gravels)
• Contributes to channel stability (in channel or on the banks)
• At the edge may create back eddies and scour pools
• Cover for fish (hide from flow, hide from predators)
• A source of food for some organisms – fungus, bacteria, some invertebrates
• Traps leaves and smaller particles of organic matter
Richardson JS. 2008. Aquatic arthropods and forestry: large-scale land-use effects on aquatic systems in nearctic temperate regions. Canadian Entomologist 140:495-509.
Fluvial geomorphology and sediment transportEast Creek, BC
Red (12 mm)
Blue (45 mm)
photos: Dr. Marwan Hassan, UBC
Wetmore SH et al. 1990. Characterization of the hydraulic habitat of Brachycentrus occidentalis, a filter feeding caddisfly. J. N. Am. Benthol. Soc. 9:157-169.
Characterising hydraulic habitat – position based on the force and turbulence of flow
Wetmore SH et al. 1990. Characterization of the hydraulic habitat of Brachycentrus occidentalis, a filter feeding caddisfly. J. N. Am. Benthol. Soc. 9:157-169.
Characterising hydraulic habitat – position based on the force and turbulence of flow
SummaryReaches are classified on the basis of their channel units (e.g. riffle – pool, step – pool, etc.)
Channel units created by tractive forces at peak flows – what remains is what resisted the forces
Large wood & boulders are the dominant structural elements in most small to mid-sized streams
Fine-scale habitat can be characterised based on structure or hydraulic characteristics