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Rivers & Streams. They are dynamic – they constantly adjust to natural and human changes Running surface water is the most important agent in modifying the lithosphere Vital human resource Impact of & cost to manage erosion & flooding is considerable. Overland Flow.
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Rivers & Streams • They are dynamic – they constantly adjust to natural and human changes • Running surface water is the most important agent in modifying the lithosphere • Vital human resource • Impact of & cost to manage erosion & flooding is considerable
What are some Factors that influence the rate of overland flow?
”Surface” Runoff (Stream Network)
Drainage Basin All the land drained by one river system
Watershed: Drainage divide High elevation points that separate 2 or more drainage basins.
Continental Divide and Mississippi Drainage A PPALACHIANS R OCK I ES watershed/drainage basin
Dendritic Uniform hardness of strata
Rectangular Pattern (Follows Joints/fractures/faults)
Trellis Pattern (Follows folded landscape)
Factors Affecting Rate of Stream Erosion & Deposition
The amount of potential energy is proportional to stream gradient.
Gradient - the vertical drop of a channel over a horizontal distance.
Streams: Headwaters The headwaters are the upper part of the stream near its source in the mountains. Upstream regions. • Steep Gradient. Lower reaches of a stream are referred to as downstream regions. • Shallow Gradient.
Streams: Mouth & Base Level The mouth is the place where a stream channel terminates and enters the sea, a lake, etc. Base level is the theoretical limit to which the stream can erode. It is, in effect, the elevation of the streams mouth.
The Un-Graded Stream Irregularities in path of river – falls, lakes, falls Rate of Deposition is not equal to Rate of Erosion
A Graded Stream: No irregularities in cross-section of river channel from its mouth to source Rate of Deposition = Rate of Erosion Streams constantly adjust channel & gradient to reach this equilibrium source mouth Base Level Lowest level to which a stream can erode Sea level
. The ability of a stream to erode also relates to stream velocity Velocity -- the speed of the water, generally measured in meters per second.
Streams: Transportation and Erosive Power • Competency: the size of particles that a stream can carry • Capacity or Load: Maximum amountof particles of a given size that a stream can carry Greater Gradient = Greater Velocity = increased competency & capacity
Velocity: straight channel The velocity of flow is greatest in the (a) middle and at the (b) top of the stream. Why? Least of amount of friction
Velocity: Curved Channel • A river erodes sediment from its banks on the outside curve and deposits sediment on the inside curve. - The Force of Moving Water
Maximum Velocity: Curve Inside of curve: Slip off Slope/point bar Outside curve: Undercutting of bank
Channel Roughness Smooth, semi-circular channel yields highest velocity Wide, shallow channel increases friction Rough channel also slows river at base
Discharge Volume of water that flows past a given point per unit time Discharge (cfs) = average stream width (ft) x average depth (ft) x average velocity (ft/sec) Example: stream dimensions are 100 ft wide and 15 ft deep ---flowing at 6 ft/sec (4 mph) has a discharge of 9000 cfs Discharge typically increases downstream: • water flows into stream from groundwater • tributaries are added downstream
Types of Stream Erosion Abrasion (aka “corrasion”) – Physical Attrition - Physical Corrosion - Chemical Hydraulic Pressure - Physical
Methods of Stream Transportation Suspension Solution Saltation Traction Load: Includes the dissolved and suspended material
PARTICLE SIZE vs. VELOCITY • Different sized materials require different water velocities to be carried via suspension, saltation, or traction. This is illustrated by the size/velocity curve…
The Upstream Headwaterregion • The collecting system • Steep gradient - high velocity, therefore high erosion. Less water = less transport • Consist of a network of narrow v-shaped tributary channels that collect water and sediment to the main stream.
The Downstream Region • The transporting system • Shallow gradient: low velocity; major process is to transport water and sediment from upstream to the streams mouth. • Consists of a wide, deep U-shaped valley with a main channel.
The Dispersing System • Consists of a network of distributariesat the streams mouth, where water and sediment are dispersed. • Upon exiting the mouth, the velocity of the stream will decrease to zero, and the sediment is deposited to form a delta(in water) or an alluvial fan(on land) • Alluvium: stream deposited sediments (sand, silt, clay)