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