The Function of Rivers
Rivers have two basic functions:
- Move runoff away from land and out to the sea (or lake)
- Transport sediments (erosion)
In doing so they help to erode the rocks on the surface of our planet, help replenish water in the oceans and help bring the raw materials to make salts to the ocean.
All rivers move at various speeds as they move towards the ocean. There are several factors that determine a river's velocity:
- Gradient, or slope - the steeper the slope, the faster the velocity
- Channel characteristics
- Shape - most channels are grouped into "U-shaped" or "V-shaped" channels. U-shaped channels are broad and shallow, while V-shaped ones are narrow and deep, although variations do occur.
- Size - larger rivers move slower than smaller ones.
- Roughness - this refers to the channel floor. As water moves over the channel floor, friction slows the water down. A smoother floor generates less friction, while a rougher one more. Water in on the surface in the center of the river moves the fastest as there is less friction to slow down the water.
- Discharge – volume of water flowing in the stream (generally expresses as cubic feet per second)
Erosion is the transportation of sediments. There are three types of load:
- Dissolved load - this refers to sediments that have been dissolved in the river water.
- Suspended load - particles that are small and light enough to be carried by the water are refered to as the suspended load. These particles are mostly silts and clays.
- Bed load - the sediment at the bottom of the river channel is refered to as the bedload. Sediments can range in size from silts and clays all the way up to boulders. These streambed sediments are collectively known as alluvium
A river's load is related to:
- Competence - the maximum particle size a river can move. Some rivers can only move smaller particles while others larger particles.
- Capacity - This is the maximum amount of sediment a river can move.
- Discharge - is the volume of water flowing in the stream (generally expresses as cubic feet per second). Capacity is related to discharge: The larger the discharge the larger the capacity.