Streams are composed of fresh running water, which has merged into a drainage system consisting of a main stream and tributaries.

The total area drained by a stream & its tributaries is called a drainage basin (or watershed).

Every drainage system collects runoff from within its respective drainage basin.

A ridge or other area of high ground divides one drainage basin from another (drainage divide).

The principal components of all streams are their channels and overbank flood plains.

Channels are described on the basis of their:

gradient: slope of channel bottom (measured parallel to stream flow direction)
cross-sectional area: equals width x depth, and increases with increasing discharge
cross-sectional shape: varies from shallow but wide (typical of streams with gravelly or sandy banks) to deep but narrow (muddy banks)
sinuousity: degree of curvature, varying from straight (S = 1.0 to 1.5) to meandering (S > 1.5)

           

Flood plains:

Broad, low relief areas adjacent to channels which become flooded when the discharge of a stream exceeds the capacity of its channels, and the stream overflows its banks.

Overbank flow deposits a sandy and silty ridge (levee) next to the channel and a fertile layer of clay and organic sediment across floodplain.

Running water is the most effective means of eroding, transporting, & depositing sediment.

Stream action, aided by mass wasting, is the dominant process of landscape development.

Stream erosion & deposition are primarily controlled by a stream's velocity & discharge.

Velocity is largely controlled by the stream gradient, channel shape, & channel roughness.

Gradient means the steepness of the downhill slope of the stream channel. In the US, gradients are typically measured in feet per mile.

As a stream moves downstream, the gradient usually decreases.
The decrease in gradient causes the stream to slow, and when the stream slows (i.e. the velocity decreases) the sediment load carried in the stream begins to drop out of the water.

When the velocity is high (or the gradient is very steep), the stream is more likely to erode the rocks or sediment over which it is travelling.

The shape of the channel also affects the stream's velocity.
The more surface area a channel has, the lower the velocity.
This is because there is friction or drag between the water in the stream & the material over which the water is flowing.

Discharge of a stream is the volume of water that flows past a given point in a given unit of time.

Discharge can be calculated by measuring the cross-sectional area of the stream & multiplying by the stream's velocity (width x depth x velocity).

Erosion -

Streams erode alluvial valleys into the land surface by downcutting and laterally eroding their channels.

Streams erode their beds by three methods:
Hydraulic action;
Abrasion; and,
Solution.

Fluvial landscapes evolve progressively through three stages of erosion: youth, maturity, and old age.

Each stage is distinguished by the:

sinuosity and gradient of stream channels
relative roles of downcutting and lateral erosion
cross-sectional shape of the stream valley

A young stream is characterized by:

rapid downcutting;
streams with straight channels and steep gradients; and,
deep V-shaped alluvial valleys with steep slopes and narrow flood plains


A mature stream is characterized by:

the start of lateral erosion and valley widening;
moderately sinuous channels with lower gradients; and,
alluvial valleys with moderately-sloping walls and wider flood plains
 

An old stream is characterized by:

extensive lateral erosion and little downcutting;
very sinuous channels with very low gradients; and,
alluvial valleys with gently-sloping walls and a broad, low relief valley floor

 

 

 

 

The evolution of an alluvial valley from youth to old age can be interrupted by tectonic uplift at any time.

This increases the relief of a region, rejuvenates its streams and restarts the sequence of valley evolution.
Rejuvenation creates erosive features such as:
incised meanders: formed when sinuous mature and old streams are uplifted and resume downcutting
terraces: uplifted and abandoned floodplains
Streams can also increase the areas of their valleys and drainage basins by the processes of:

headward erosion: the erosion and retreat of their upper drainage divides

stream piracy: occurs when headward erosion enables one stream to capture the water flow from a second stream


Deposition -

Streams carry three main groups of sediment downstream:
Bed load
Bed load travels on or near the stream bed.
Coarse or dense sediment is carried by traction & saltation (sliding, rolling, or bouncing).

Suspended material
Fine-grained material, such as silts and clays, are suspended & carried within the water column.

Dissolved material
Substantial material, typically mineral constituents dissolved out of sediment, can be carried within stream water.

Streams tend to change their shape over time in an effort to reduce their gradient and slow their velocity.

The cross-section of a young stream is typically a "V" shaped valley, with the stream channel occupying the base of the "V", and erosion outweighs deposition.

The stream develops meanders, or curves, as it matures, and eventually deposition overtakes erosion.

Streams that have low gradients and velocities deposit large amounts of sediment and become "braided" as they age even more.
Large amounts of sediment can also be abruptly deposited if a large change in gradient occurs over a very short distance, such as when a stream reaches a valley floor (i.e. an alluvial fan) or an ocean (i.e. a delta).

The three major continental environments of stream deposition are:
alluvial fans
braided streams
meandering streams
streams also deposit sediment at river-mouth deltas along ocean coasts and lakes

Alluvial fans:

cone-shaped accumulations of sediment at the bases of mountains

surface is characterized by a radial drainage pattern of distributary channels

distributary channels split into progressively smaller channels from inner to outer fan

flow velocity in distributary channels also decreases from inner to outer fan

Braided streams:

characterized by several channels and channel-bottom bars

stream channels split and rejoin around channel bars to form a braid-like pattern

channels are straight to slightly sinuous, and wide and shallow in cross-section
Meandering streams:

consist of single meandering channel which flows through a broad, low-relief flood plain

channel erodes cut-banks on outer meander bends, and deposits sandy point bars on inner bends and silty levees along its banks

abandoned meanders form oxbow lakes

Hazards

Streams are excellent sources of fertile soil for agriculture, fresh water for industry & consumption, and transportation, so many large cities are built near streams--even on their banks--and are therefore threatened by periodic flooding.

Flood plains are easily identifiable landforms.

Flooding doesn't typically happen every year, but does regularly reoccur on every stream.
Moderate floods occur about every 10 years; major floods occur approximately every 100 years.
A 100-year flood is one that occurs, on average, every 100 years.

The reoccurrence interval describes the average time between floods of a given size.

Recurring, seasonal floods occur in many regions of the world, and can be predicted with a small amount of uncertainty.

Engineering controls, such as dams, levees, & dikes, can aid in preventing damage from smaller flood events, although may provide little help in larger or flash floods.

Typically, water in a stream valley is well below the level of the stream banks, but in times of high discharge, water may overflow the stream banks and cause a flood.

Many factors control flood severity, including the quantity of water, rate of water entering the stream system, rate of surface runoff (including whether an area is impermeable or paved), topography, and vegetation.

Flood prevention and management is aided by restrictive zoning, diversion channels, levees, dams, and reservoirs.