Chapter 10: Stream Erosion and River Systems

4 processes that transport sediment

(1) solution (2) suspension (3) traction (4) saltation

Multiple-Thread Channels

- anabranching channels - braided stream

Trellis

- common in dipping or folded topography - develops in response to underlying structures that alternate bands of tilted hard and soft strata - long parallel streams linked by short, right-angled segments

types of drainage patterns

- dendritic - trellis - radial - parallel - rectangular - annular - deranged

features of floodplanes

- flanking many stream channels are flat, low-lying areas called floodplains - formed by erosion, but surfaces reflect centuries of flooding and deposition - natural levees --> when water spreads out, it drops some of its sediment load and forms natural levees

Stream erosion processes

- headward erosion - downcutting - lateral erosion

alluvial fans

- in arid and semi-arid environments - mouth of canyon where an ephemeral stream exits into a wider valley

Stream types

- perennial streams - intermittent streams - ephemeral streams - exotic streams

Types of single thread

- straight channels - meandering stream

Stream dynamics

- stream gradient - degree of inclination

factors that influence erosion and deposition:

- stream gradient (slope) - base level (lowest point to which it can flow) - discharge

Stream size, velocity, and discharge

- stream width, depth, and velocity increases as discharge increases - discharge increases as distance downstream increases - stream width, depth, and velocity increases as distance downstream increases

Drainage basins...

collect water and sediment from many tributaries - all streams start at headwaters (upper tributaries of a river)

traction

current literally drags bed load (coarser grained materials)

lateral erosion

currents in meanders swing from side to side

headward erosion

cuts into interfluve at upper end of valley

most common drainage pattern

dendritic - geology is uniformly resistant to erosion

river deltas

depositional plain that forms at the mouth of a river, triangular in shape

solution

dissolved load mostly derived from chemical weathering

properties of low drainage density

easy soil infiltration

cause of nickpoints

erosion undercuts more resistant strata to erode weaker strata until the more resistant strata is pulled down by gravity

braided stream

excess sediment and stream might become a maze of interconnected channels - formed when reduced discharge lowers a stream's transporting capability (after flooding or near mouth of stream)

alluvial deposits in floodplains are good for...

farming

suspension

fine-grained particles held aloft in the stream, with finest being deposited last when the stream slows down

Perennial streams

flow all year

intermittent streams

flow for several weeks or months and may have groudnwaterinput

ephemeral streams

flow only after precipitation and are not connected to groundwater

drainage divides

form on higher land and separate drainage basins - continental divides - mountain ranges separate drainage basins in US and Canada

alluvium

general term for clay, silt, sand, gravel, and mineral fragments deposited by running water

10-year flood

has a 1 in 10 chance of occurring in a given year - high probability, lower threat

100-year flood

has a 1 in 100 chance of occurring in a given year - lower probability, higher threat

interfluve

high ground that separates two river valleys

flood

high water that overflows the natural bank along any portion of a stream

2 causes of stream erosion

hydraulic action & abrasion

Stream discharge is graphically displayed on a

hydrograph

aggradation

if bed load and suspended load exceed capacity, sediment accumulates and stream channel builds up via deposition

where is hydraulic action maximum?

in upstream tributaries of a drainage basin where sediment load is small and flow is turbulent

graded stream

just enough energy to transport sediment load

anabranching channels

large river systems and multiple large channels are present across a vast floodplain

base level

level below which a stream cannot erode

downcutting

lowering of the streambed

steps to create oxbow lake

meander --> neck narrowing --> cutoff forms --> oxbow lake forms

nickpoints

occur when longitudinal profile of a stream experiences abrupt change in gradient - waterfalls (large) - rapids (small)

saltation

particles bounce along the bottom because they are too large to be picked up

laminar flow

particles move in the same direction

straight channels

perfectly straight channels are rare, but many streams in steep mountain regions or in bedrock controlled channels are relatively straight

properties of high drainage density

precipitation can't infiltrate soil; overland flows form a dense network of streams

calculation of flood risk

probability based on a time interval based on historical data

Radial drainage

results when streams flow off a central peak or dome

Interfluves direct sheetflow into

rills and gullies and into streams

network of tributaries

river system

abrasion

rock particulates grinding and carving the streambed like sandpaper during transport

ultimate base level

sea level - local base levels can keep from eroding to sea level (lake, hard rock, reservoir)

gullies

small channels, but larger than rills

stream

smaller than a river

rills

smallest channels

finer to coarser materials

solution --> suspension --> saltation --> traction

hydraulic action

squeeze and release action of flowing water that loosens and lifts rocks

Competence

stream's ability to move particles of a specific size - a function of velocity and available energy

fluvial

stream-related processes

hydrology

study of water and its global circulation, distribution, and properties - surface water and groundwater hydrology

Drainage patterns

the arrangement of stream channels in an area

drainage density

total length of all stream channels in the basin/area of basin

Capacity

total possible sediment load a stream can transport - a function of discharge

2 types of flow cause stream erosion

turbulent flow and laminar flow

stream discharge/runoff

volume of water moving past a point in a given unit of time - Q = V*A

turbulent flow

water particles move randomly and cross paths in many directions

meander

when channel slope is gradual, streams migrate across valley bottoms in a snake-like pattern - undercut bank - point bar - oxbow lake

Stream-Capture

when one stream intersects another due to headward erosion and changes the stream direction - elbow of capture --> captor, captured, beheaded streams

confluence

where 2 tributary streams join together

flood hazards occur...

because hundreds of millions live along floodplains

exotic streams

begin in moist locations and flow downstream into arid landscapes

Meandering Stream

channel slope is gradual and streams develop a sinuous form

Drainage basins/watersheds

Each waterway has a drainage basin, or watershed, which is an area of land where all precipitation that falls will drain or flow downhill into a specific stream.

Upstream --> Downstream processes

Erosion --> Transport --> Deposition

river

a main stream

tributary

a river or stream flowing into a larger river or lake

ungraded stream

actively eroding or depositing material in its channel so that the gradient is adjusted to the sediment load