A river is a stream of fresh water that flows towards a lake or an ocean. It is a complex structure, much more than a line of water moving across the land. As water moves through a landscape, the land changes the water and the water changes the land.
What is a River Made Of?
Rivers are made of water, of course. However, under the river water is a substrate, called the river bed. The river bottom may be made of many different things, depending on where the river is located. It may be made of mud, sand, different sizes of rocks, and pieces of wood. The river may pass over existing rocks underneath, called bedrock. It may pass through wood that has fallen from the trees around the river.
Rivers Have Areas of Fast and Slow Moving Water
Generally, a river has areas of slow flow and fast flow. The areas of slower flow are called pools, while the areas of fast flow are called riffles. In a riffle, the water bounces across rocks, adding oxygen. It often looks white in these areas because of all of the bubbles. In a pool, the water deposits sediment and leaves. Fish and other animals find places to hide in these quiet areas.
Rivers Are Not Straight Lines
Rivers move across a landscape, but they do not move in a straight line. They meander back and forth. The flow of water moves as the water moves around disturbances. Perhaps a large rock that is difficult to erode is in the way. The water follows a path around the rock and moves towards the opposite bank. As the water moves into the bank, it takes some of the bank away with it. This process is called erosion. In areas of slower flow, the river deposits some of the sediment it eroded from other banks. The area of strongest flow and deepest scouring of the river bed is call the thalweg. This is the natural direction of the river.
Water Changes Its Surroundings and the River Bed
Water actively changes the place that it moves through. A river carries rocks as it moves along. Sometimes these rocks are sediments that are dissolved or suspended in the water column. Other times, when the river is in flood, it bounces and rolls much larger rocks along the stream bottom. When a river carries and rolls rocks, this is called traction. When it bounces rocks along the river bottom, this is called saltation. At some point, these rocks are deposited at another point in the river and become part of the river bed.
Rivers are an amazing part of the landscape, moving and changing the places around them. Next time you stand beside one, watch it and look at the complex ways in which the water and land around it interact.
Sources:
Bridge, J.S. Rivers and Floodplains: Forms, Processes, and Sedimentary Record.
Blackwell Publishing, 2003
 |
