Physical Geology
River and Streams
River Systems:
- 3 major subsystems:
- 1) collecting system
- - river tributaries collect and funnel water and sediment into the main river channel
- - tribs usually form a dendritic, or treelike pattern w/ numerous branches
2) transporting system
- - the main trunk of the river, through which the water and sediment is moved away from the collecting area toward the ocean.
3) dispersing system
- - distributaries at the mouth of a river where sediment and water are dispersed into the ocean.
Dynamics of river systems:
rivers are complex systems influenced by number of things, which are:
- - discharge
- - velocity
- - gradient
- - sediment load
- - base level
1) Discharge
- amount of water passing by a given point during a specific time interval
- measured in cubic meters/second
- flow of river comes from runoff and from groundwater seepage
- groundwater is most important factor in supplying rivers with water.
2) Velocity
- not uniform through a stream channel
- depends on shape and roughness of the channel
- steep gradients produces faster rates of flow, also the greater the volume of water, the faster the flow.
- on a curve, the fastest water is on the outside, where the water brushes up against the far bank, causing erosion (cut bank)
- on the inside, velocity is much reduced, allowing some sediment to be deposited, forming point bars.
3) Gradient
- steepest at the headwater in the collecting zone
- decreases as one travels to the mouth of the river, where the gradient is almost zero.
- forms a nice concave cross-sectional view of a typical river, from head to mouth.
4) Sediment load
- fine particles are moved in suspension, never toughing the ground (suspended load)
- - silt & clay giving the water a muddy look
- coarse particles are moved by traction along the stream bed, jumping along (bed load)
- - move by saltation: short leaps as a strong current picks them up and moves them a short distance.
- - 7-10% of the total sediment load, on average.
- - abrasion between particles causing them to wear, smooting them and forming round pebbles and stones.
- - also wears away the stream channel along the sides and bottom of the channel.
- dissolved materials are carried along in solution (dissolved load)
- invisible chemical ions
5) Base level
- the lowest level to which a river can erode its channel
- its the elevation of the stream's mouth, where it enters an ocean, or lake, or another river.
- cannot erode below this level, or else the river stops flowing at that point, causing water and sediment to pool at that point, forming a lake.
- eventually, water would overflow the lake and coninue travelling to the mouth.
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