Stream Processes

Fluvial geomorphology

• Alteration of land by streams

• ways rivers impact shape of earth

How can rivers impact the shape/ topography of earth

• erosion and deposition

More water in a stream means

more sediment

Slower water results in

more deposition

change to land or water causes change in

stream system

Characteristics of rivers

• self-sustaining( they will adjust as surroundings change)

• retains the same general geometry over decase

• maintain a Balance between sediment import and export

Increasing water conveyance capacity of a stream

• get more water out faster so it doesn’t build up and flood

• making the banks concrete

Consequences of Increasing water conveyance capacity

• The river is no longer allowed to adjust

• floods in an unnatural way in the unpaved areas

  • Heavy flooding and erosion downstream

• Concrete doesn’t last forever

Basin Size increases

downstream

Basin starts as

rills, to gullies, to small channels

what can cause channel size increase

depends on amount of sediment and water

Stream Order

—Means of comparing rivers of different size or importance

Strahler method

• First order: no tributaries

• Second order: joining of two first order streams

  • 1+1=2

• Third order: two second order streams come together

  • 2+2=3

in strahler stream order method Order number should be proportional to

—Channel dimensions

—Watershed size

—Stream discharge

Shreve Magnitude Stream Order

• Considers streams as links

• Magnitude of each “link” is sum of all tributary values

  • 1+1=2, 2+1=3

  • Value represents number of first-order streams upstream from a given point

• Used by rainfall-runoff estimators

Drainage Density

Length of all stream channels in drainage basin divided by total basin area

High drainage density

• Generally distribute the flow more evenly

• Little lag time to low-order streams

• Greater lag time to downstream flooding

higher stream order indicates

high drainage density

Why would you want to know drainage density

tells you how the water is getting into our river

Low drainage density

—Channels carry proportionately more water

—Tend to be flashy

Drainage Patterns depend on

—Shape of landscape

—Composition of underlying material

Dendritic

Radial

Rectangular

Trellis

Parallel

straight channel pattern

• Least common natural form

  • unstable

Meandering channel patterns

—Most common natural form due to inherent flow structure of water and roughness elements

—Sinuosity

Braided channel

• The channel has multiple internal splits and rejoins

• Typically has high sediment load, highly variable discharge, and erodible banks

  • Proglacial zones, arid environments, and urban areas

    • seen in arid places

pt. bar

area of sediment deposition

Thalweg

line connecting deepest points

Cut bank

erosional area, often with overhanging edge

• river speeds up faster on the outside of curve

As a river meanders it will

• erode the bank materials and when the river moves the flood water sits in the flat eroded area

pools

finer sediment, low gradient, deep

• greater depth and low velocity

riffles

• coarser sediment, high gradient, shallow

• large volume

• faster( high gradient)

• provides roughness

turbulent flow

• changes in velocity and how water interacts with things around it

Helical flow

• corkscrew

• carves away

• more erosion more migration of the river

Channel Shape is dictated/self formed by

the amount of water and sediment available

What does it mean that channel shape is self adjusting

• Adjusts to changes by humans, climate, vegetation

• Carve deeper if more water

• Become more shallow if less water

Cross-section of a river

—Gently rounded

—Sometimes trapezoidal with straight sloping sides

Cross section of large rivers

• much wider than deep

Cross section of small rivers

can be much deeper than wide

—Shape depends on interaction between:

—Factors related to debris load (size, lithology, amount, depositional forms)

—Factors related to water flow

Bankfull

• most effective time for river to change shape

• when water fills up to banks

River Continuum Concept (RCC)

• how geomorphology and river location impacts biological communites