River Erosion and Deposition
Rivers are the main pathways for water runoff as it travels from areas of high to low elevation.
Rivers do most of the work in shaping many of the landscapes we see around us.
Rivers and their branches (known as tributaries) form a connected network that:
- erode material from some areas and
- deposit it in other areas such as lakes, ponds, and ultimately into the ocean.
Exposed crust leaves a variety of minerals available to be broken down into loose particles and then carried away.
Mminerals include common silicates we see in rocks (quartz, feldspar, pyroxene, and amphibole).
Once physical and chemical weathering create this material, it is washed into rivers by smaller streams.
Starting Steep ...
Gravity is a major factor as rivers begin at higher elevations in hilly or mountainous country, where they develop channels that are narrow, fairly straight, and have fast flow due to the steep slope.
And the faster a river flows, the heavier the material it can carry:
- So faster flowing rivers would commonly carry more sand, gravel, and even small boulders!
- Slower flowing rivers tend to carry smaller grained sediment such as silt and clay.
The amount of material a river carries also depends on its size, and the amount of water it can carry (known as the discharge).
Rivers erode their channels making them continuously deeper and wider over time.
Some material carried by rivers just simply bounces along the river bed and is therefore called bed load.
Some particles are small enough to be carried along and held up by the flowing water. This material is called suspended load.
Also some minerals can dissolve in water, and can be carried this way "in solution."
Slowing down ...
Once rivers reach lower, flatter areas, they tend to slow down and widen. While they continue to erode and carry sediment toward the ocean, it is in these areas that they also do some deposition (letting some of the material drop out).
Further on, where they enter larger water bodies, such as the ocean, they create what are perhaps their most recognized depositional features - deltas.
The Indus River Delta
Wide sweeping curves in in rivers are called meanders.
Meanders begin just as rivers start to slightly bend. They erode material from the outer bank (the cut bank) of the channel and deposit on the inside (called the point bar). In this way the curve of the channel grows and takes on a wider, circular path.
With more erosion, the outside portion of meanders are deeper. These areas are called pools. The bank on the outside is called the cut bank.
The inside bank which grows from deposition is called the point bar. Rivers also deposit along the straighter sections between meanders (areas called riffles).
The difference in path between a younger and older river.
It develops wider turns and grows outward more and more
until it cuts off from the main channel and becomes a separate lake.
This lake is known as an oxbow lake, based on its shape.