STREAMS

Streams

• Streams - all water flowing in a channel, regardless of size
• Physical weathering agent
• Important for transport and deposition
• Ability to erode and transport sediment depends on velocity

Stream Flow

• Stream flow - laminar versus turbulent
• Laminar flow - streamlines (lines of flow) do not cross
• Turbulent flow - streamlines cross, contorted
• Slow, shallow flow and smooth bottoms favor laminar flow

Stream Velocity

• Factors Controlling Stream Velocity
• Gradient - slope along stream as well as between streams

• Shape - most efficient is stream with smallest perimeter for a given cross-sectional area
• Which stream has the highest velocity? Why?
• Size - an increase in size reduces perimeter/cross-sectional area ratio. Effect?
• Roughness - rougher bottom (boulders, cobble, etc) slows stream velocity by disrupting flow and causing turbulence
• Discharge - amount of water flowing past a point in a given amount of time
• discharge (m³/sec) = cross-sectional area (m²) x velocity (m/sec)
• Ex. Mississippi River 17,700 m³/sec; Amazon discharge is ~ 10x greater

• If discharge increases:
• Channel width and/or depth increases
• Velocity increases

Downstream Changes

• Downstream changes in a stream
• Gradient decreases
• Discharge increases . . . tributaries
• Width, depth and velocity increase

Sediment Transport

• Competence of stream - measure of largest particle stream can carry
• Capacity - total amount of sediment it can carry
• Sediment transported by Streams
• In solution - dissolved load
• In suspension - suspended load
• Along bed - bed load
• Grain size transported depends on velocity, water depth, and cohesiveness

Velocities Needed to Transport (Suspend) Grains (p. 293)

Sediment Transport (cont.)

• Suspended load accounts for the greatest proportion of load
• Ex. Mississippi River
• Suspended load = 67%
• Dissolved load = 27%
• Bed load = 7%

Base Level

• Base Level -the lowest point to which a stream can erode its channel

Graded Stream

• Graded stream - hypothetical equili-brium state, no erosion or deposition; longitudinal profile remains constant.

Stream Channel Patterns

• Three kinds of channel patterns - meandering, braided, and straight.
• Meandering
• Low gradient
• Deposition on inner bank (Point bar), erosion on outer bank (Cut bank)

• Process of erosion and deposition will result in meanders migrating downstream

• Features
• Valley walls
• Floodplain
• Yazoo stream - tributary that follows parallel to the main channel until a breach in levee occurs

Formation of Levees

• Levees - sediment buildup near riverbank formed from deposition of coarse sediment during flooding

• Coarse sediments deposited near river bank

Formation of river-valley deposits by a meandering channel (transparency)

• Braided - streams with numerous anastomosing subchannels which are continually dividing and reuniting.
• Straight

Stream Channel Patterns
Importance of Slope and Discharge

• Geomorphic Features
• Oxbow lake - lake formed from cut-off meanders (should have been mentioned earlier)
• Alluvial fans - delta-shaped deposits formed from deposition of sediment after an abrupt decrease in stream velocity

Geomorphic Features

• Sediment deposited due to a decrease in stream velocity is called alluvium
• Delta - sedimentary deposit formed when a stream or river enters relatively still waters (lake, ocean)

Drainage Systems

• Drainage systems and patterns
• Area drained by a river or stream = drainage basin
• Drainage basins are separated by topographic highs called divides.

Drainage Patterns

• Dendritic (Fig. 14-26a)
• Irregular branching (tree-like)
• Uniformly eroding surface
• most common type
• Trellis (Fig. 14-26b)
• tributaries nearly parallel
• forms in area underlain by alternating bands of resistant and less resistant rocks
• Rectangular (Fig. 14-26c)
• stream follows jointing pattern
• Radial (Fig. 14-26d)
• Divergence from a central topographic high (dome, volcano)

Stream Development

• Youthful
• down cutting
• rapids, waterfalls
• v-shaped canyon
• little or no floodplain
• Mature
• floodplain, meanders
• valley walls
• Old age
• floodplain several times wider than meander belt
• no visable valley walls
• natural levees and yazoo streams a common feature.

Other Features

• Incised meanders - rejuvenation (uplift) causes down-cutting of meanders
• Superposed stream - existing stream cuts across "emerging" ridge