Stream Geomorphology

Watershed

• The line that demarcates the topographic “shedding” of the water across the landscape.

Catchment

• The area of a landscape within the watershed boundary that routes water to the river network.

Basin Characteristics

• Drainage Area– Area from ridge to ridge that contributes to the water

supply of the stream– Usually determined from topographic maps

• Drainage Density– Length of perennial channels divided by drainage area

Stream Network- Order

• Stream order (Strahler 1952)– Perennial streams without tributaries are termed first-

order– When two streams of equal order come together, the

downstream reach is increased one order

Stream Network- Link Magnitude

• Link Magnitude (Shreve 1966)– Each junction is a link– Link magnitude is the sum of the links

• Exterior link magnitude includes the lower channel (n)• Interior link magnitude is 1 less than exterior link

magnitude (n-1)

Hydraulics

• Discharge: volume of water passing a point per unit time – cubic feet per second (cfs) or cubic meters per second

(cm3/s)

Q = V * A Q = V * width *depth

• Velocity: rate of movement of water – meters per second (m/s) or feet per second (f/s)– mean velocity occurs at 60% of the depth

Hydraulics- Velocity profile

Measuring velocity

• “Float the orange”: record the amount of time it takes an object to travel a measured distance

• “Spinning cup”: records number of revolutions made by a series of rotating cups (like a horizontal paddle wheel)

• Electronic sensor: submersible sensor records differences in pressure

What Affects Velocity?

• Depth• Slope• Wetted perimeter• Hydraulic radius • Roughness

Uniform Flow Theory

Manning equation (1889)

1

V = R2/3 S1/2

n

V = Velocityn = Manning’s Roughness coefficientR = Hydraulic radiusS = Slope

Uniform Flow Theory

• Manning Equation solved for discharge (Q)1

Q = A R2/3 S1/2

n

Q = DischargeA = Cross-sectional Area

Manning’s n

• Measures channel roughness• Assumes:

– Uniform channel configuration, which assumes:• depth, cross-sectional area, and velocity are uniform

throughout the reach• water surface and streambed are parallel throughout

the reach• Which means:

• These assumptions are net met in natural channels!!!!

Manning’s n

• Range of Manning’s n:– Between 0.008 (glass) and 0.2 (many

obstructions)– Commonly between 0.025 and 0.1

• Typical Manning’s n in natural channels– gravel, cobble and boulders n = 0.040– cobble with large boulders n = 0.050– floodplain with trees n = 0.12

Channel Cross Sections

• Area (A): cross-sectional area of flow (m2)

• Wetted Perimeter (WP): length of wetted contact between water and the stream bed along a cross-sectional transect (m)

• Hydraulic Radius (R): Area/Wetted Perimeter

• Velocity (v): rate of water flow (m/s)

• Discharge (Q): AV (m3/s)

• Slope (S): Change in elevation over distance (m/m)

Substrate

• Wentworth scale of substrate size:– Boulder: greater than 25.6 cm– Cobble: 6.4 - 25.6 cm– Gravel: 0.2 - 6.4 cm– Sand: 0.006 - 0.2 cm– Silt: < 0.006 cm

Valley Landforms

• Drainage network- total valley floor within a basin

• Section- length of valley that represents a common landform origin and often is topographically delineated– Example

• Montane section• Lowland valley• Marine terraces

Valley Landforms- Reach Scale

• Reach Type- Length of valley influenced by adjacent hillslope landforms

– Constrained Reaches: valley width <2 active channel widths

– Unconstrained reaches: valley width >2 active channel widths

Valley Landforms- Valley features

• Terraces- upper surfaces which may be of fluvial origin and are formed during extremely rare events

• Floodplain – fluvial surface created by floods that occur less frequently than average flood discharges (decades to centuries)

• Active channel – area of channel being actively modified by average stream discharges

Valley Floor Features

Terrace

Floodplain Active Channel

Wetted Channel

Describing the Channel

• Longitudinal Profiles• Valley Landforms• Valley Floor Components• Channel Units• Channels Subunits• Channel Cross Section

Longitudinal Profiles

• Diagrammatic representation of change in elevation with distance

• Steeper gradients in the headwaters– Gradient expressed as percent or degrees (100%

= 45 degrees)

Channel Characteristics

Longitudinal Profiles

The Active Channel• Active channel (Bankfull Width)– area of channel being

actively modified by average stream floods

• Average stream floods occur approximately every two years in most basins

• Equals the wetted channel plus the active channel shelf– Wetted Channel: area of wetted stream bed– Active channel shelf: portion of channel exposed at low

flow but inundated at normal high flow

Channel Units

• Channel Unit: Length of active channel that represents common process of bedformdevelopment at high flow

• Each unit must be longer than one active channel width

Channel Units - Pools

• Low gradient• Little surface turbulence• No supercritical flow

Channel Units- Riffles

• Slightly higher gradient• Turbulent water surface• Supercritical flow less than 10%

Erosional or depositional?

• Pools:– Depositional at low flow– Erosional at high flow (and over time)– Areas of low velocity at low flow and high velocity during

floods

• Riffles:– Erosional at low flow– Depositional at high flow– Areas of high velocity at low flow and areas of low

velocity during floods

Erosional or depositional?

Summary

• Q = VA• Velocity is affected by many things• Velocity affects many things• Stream channels are dynamic