Week ten Flashcards
Why study streams?
Streams provide water and deposit fertile soil for
agriculture.
They are pathways for commerce and trade.
They also flood, erode, and sculpt the landscape.
Streams are everywhere and many cities are adjacent
to them.
North America: use water from streams and lakes—
one trillion liters per day
(1,000,000,000,000 liters/day)
Where does the water come from?
Ø Water moves through the Earth system along pathways in the
hydrologic cycle. Evapotranspiration moves water into the
atmosphere; precipitation brings it back to Earth. Once on the
ground, it may flow across the surface as runoff (into stream
channels) or infiltrate down into the subsurface and become
part of the ground water.
Ø A drainage basin is the area that drains into a stream. Basins are separated by ridges called divides. Larger streams (like major rivers) contain the drainage basins of all the tributaries that feed into it. Ø Major drainage basins of the North American continent. While the Mississippi basin is the largest river basin, the basins in Canada that feed Hudson Bay and the Arctic Ocean are indeed sizable. The Great Basin, west of the continental divide, does not drain into a sea or ocean, but instead exits into lakes and playas
Discharge
is the amount of water that flows through a channel. Discharge is calculated by first finding the cross-sectional area of a stream and then multiplying this times the velocity.
Where does sediment come from? Hint: particles
- Particles loosened by weathering are picked up in surface runoff and transported to the stream channel.
- Mass movements events may move loose material downslope into the stream channel
Sediment moves as bedload and suspended load
Bedload: large grains that cannot be picked up, but still are
able to be moved. They roll, bounce, and slide along the
bottom
- When a stream is at bankfull, all sizes of particles typically move due to increased stress.
Suspended load
small sediment grains that mix with the flowing water are transported above the bed, rarely touch bottom, and can make the water cloudy or muddy.
Dissolved load
the hidden load Ions in solution from chemical weathering are also carried by water in streams.
How do streams pick up sediment?
Ø A stream must pick up sediment grains to move them downstream.
Ø Both picking up and moving particles requires work and expends energy.
Ø Motion is sufficient energy to move particles.
– Eroding the stream bed also requires energy; does the stream have this much energy as well?
An alluvial stream
- Where the channel is in water - transported sediment ) has a different energy structure than a bedrock stream that is cut into solid rock
How shear stress moves sediment
Force, not velocity is exerted on a particle to move it. Moving water has a shear stress parallel to the direction of motion and depends on the weight of the water and the steepness of the slope, which in turn, affects acceleartion.
Shear stress
Shear stress increases with increasing water depth (increasing mass acting on the particles below). Also increases with increased slope. In either instance, the shear force must overcome the resisting forces of friction and cohesion that keep the particles in place.
Shear stress vs, particle size
The smallest particles take a much shear force to dislodge as the largest. Small particles of silt and clay actually have large cohesive forces for their size that keeps them in place. However, once moved, these particles tend to stay in the flow
The suspended load can stay entrained in the water where turbulence keeps them until they reach a condition that deposits them.
Stream power
- The ability of a stream to do work. Commonly measured by multiplying shear stress times the average velocity.
- Stream power is consumed moving the load. If there
is excess power in a stream, then it may erode more materials from the stream bed or bank. If stream power is insufficient, then deposition occurs until load matches power.
Why do streams deposit sediment?
- A stream must lose power to move sediment when it
switches from erosion to deposition.
Ø Stream power is the result of shear stress and
velocity.
– Loss of shear stress will cause deposition
.
– Loss of velocity will cause deposition.
– Loss of discharge will cause deposition, because without
water there is no shear stress, nor any velocity!
Alluvial fans
Deposition happens where water depth decreases and stream power drops off. A stream in arid landscapes where depth goes to zero very rapidly (like when a stream empties out onto a valley floor) results in an alluvial fan that forms where the stream goes from confined to unconfined
What factors determine channel pattern?
Two basic patterns
Meandering: a single main channel that has various loops and/or curves down its length
Braided: a series of small channels that are interwoven into a larger whole
What factors determine channel pattern?
Two basic patterns
Meandering: a single main channel that has various loops and/or curves down its length
Braided: a series of small channels that are interwoven into a larger whole
The role of banks in determining channel pattern
Bank erodibility influences channel pattern.
– Easily eroded material tends to make a wider, shallower stream, leading to a braided pattern.
– Where the banks are more stable because of clay or vegetation, the channel tends to be narrow, deep, and sinuous.
Why Do Streams Flood?
Ø Discharge that exceeds a stream channel’s
capacity is due to greater than normal precipitation.
Ø Floods occur when a drainage basin cannot absorb the water from precipitation or snowmelt and it must then run off onto the surface. Ø High-precipitation events can lead either to brief flash floods if of short duration, or to longer duration floods that may last days or weeks
Floods: Causes
. Extreme rainfall. Precipitation rate causes excessive runoff from land surface, which exceeds bankfull capacity of the channel.
– can lead either to brief flash floods if of short duration, or to longer duration floods that may last days or weeks
Flash floods
Ø Flash floods associated with canyons are generally the deepest and often most damaging.
Ø These events are rapid (flash) and difficult to predict
