Rivers and Streams Lecture Notes

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:

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.