Cross Section Of River Zones

Side View Of A River

Top Down View Of A River

Cascade

When water flows over larger rocks and boulders, it becomes a cascade. Cascades are rough places to live. If you've seen one, you know that water often beats onto rocks so hard that it creates foam and spray. But there is a lot of life in cascades because of the high oxygen levels.

Falls

As harsh as the environment is in a cascade, it is even more brutal in a falls. But like the cascade, the water is highly oxygenated. Diatoms are able to cling to vertical services quite well. Any animal that wants to live in a falls is going to have to be able to perform the same feat. After all, as the water drops, it is going very fast.

Pool

Pools provide darkness and slow movement of water. Their oxygen levels are often low. During droughts, pools are sometimes the only parts of a stream that stay wet. The stream's water suddenly slows down, dropping its load of silt and organic materials.

Embayment

Embayments are habitats created when a stream's water flows by boulders or woody debris. The water rushes around the obstruction, carving out a small bay in the side of the river. Embayments may not look like much, but their flow characteristics make them very special. Water that enters from the main channel eddies, flowing around in a circle, before exiting back into the main channel. They are almost closed systems. The water that is eddying around inside the embayment stays long enough, and slows down enough, to drop its load of sediment and organic materials. Stream plants and algae grow well in these slower, richer areas.

Run

In large rivers most people think of long, smooth-flowing stretches of fast water. These are called runs. Smaller streams have them, too, of course. The surface may seem smooth, or it may have whirls of turbulence--but there is no froth or spray.

Plunge

Plunges, also known as plunge-pools, are a lot like pools, and provide habitat for many of the same animals. They develop when water flows over an obstruction, for instance a very large boulder sunk into the streambed. Water rushes over the obstruction and downward, carving out a long, narrow pool in the streambed. Sometimes these are out in the middle of the stream, and can't be seen (until you fall into one!)

Just like a pool, a plunge offers a deep, dark place for larger fish to hide. Drift travels over the lip of the plunge, making it an ideal place to wait for food. Each plunge also has a tail, a long, shallow place at its downstream end. The tail concentrates drift (food) that has entered the plunge, so it is also a good place to feed.

Riffle

Riffles are a rich part of any river.

Backwater

A backwater is a place where the water of a stream has found its way out of the main channel, and where there is an inlet but no outlet. This can happen when boulders or large woody debris cause sandbars to form. It can also happen when side-channels in a stream get cut off on one end. The water in a backwater may be shallow or deep. It is very slow-moving, even stagnant. Because the water moves slowly or not at all, for most of the year it is warmer than the main stream. The slow movement causes fine sediments and organic materials to accumulate on the bottom.