A river is a body of water with current moving in one general direction. They can vary in size, with smaller versions of rivers being referred to as streams, creeks, or runs. The water in a river flows into either a larger river, a bay, or ocean. Rivers themselves are fed by smaller rivers or streams. Rivers are important habitat to many different types of plants and wildlife.
Below are some characteristics of rivers.
Some rivers are narrow and others are very wide. Even the same river can be narrow at some points and wide at others. Overall, a river tends to be smaller at its beginning, called a "headwater," and widest at its end, where it empties into a larger body of water.
Rivers also vary greatly in their depth. A smaller river may be very shallow (only ankle deep) at some parts. Large rivers can be dozens of feet deep. River depth will often decide what types of life the river can support, since some plants and animals prefer shallow water, while others prefer deep water. Rivers also have "holes," which are deep spots in an otherwise shallow river. These holes are important for larger fish to hide in, but they are also dangerous to people who aren't careful wading across a river swimming.
The speed of the current is also important in determining what kinds of life the river supports. Many small animals, such as some types of aquatic insects, cannot keep from being swept away in a fast current. Others, like dobsonfly nymphs, specialize in fast water. Many fish and other animals love deep, slow-moving water.
All rivers and streams "meander." This means they have bends and turns, depending on the geography of the land. Land that is fairly flat may have a straighter river, while hilly land probably creates a river that meanders more. A "switchback" is when a river doubles back on itself, making a "U" shape. Bends in rivers also affect depth. The outer part of a river bend is usually deeper, while the inner part of the bend is usually shallower.
Have you ever seen a river where some of the water looks like it is moving the wrong way? This is an eddy. Eddies are spots on the river, usually at the edges, where some of the water moves back upstream. This countercurrent usually moves slower than the main current, and if you watch closely, you'll see the water is actually moving in a big circle and eventually continues downstream. Eddies tend to be deeper than other parts of the river, and many fish and other animals live there.
The banks of a river may be gently sloping to the water, with a flat beach; or, there may be a steep drop, or even a cliff. An "undercut" bank is usually on the outside part of a river bend. With an undercut bank, water is actually "cutting" away soil underneath the bank. Water may go several feet under the bank, even below where you are standing. Fish, muskrats, and other animals use undercuts as shelter.
The type of bottom can vary. Fast-moving rivers, or parts of rivers, usually have a gravel (small rocks) bottom. Slower-moving rivers tend to have mud bottoms. The same river may have both depending on the stretch of river.
Rocks and Boulders:
Many rivers have lots of rocks or large boulders in them. If a large rocks sticks up out of the surface of a river, then the current has to go around it. Holes and eddies often form behind large boulders.
Just like ponds, rivers support a wide variety of plant life. Some of the same plants found in ponds are also found in slow-moving rivers. Other plants prefer shallow, fast moving water.
Many different animals live in rivers, while others come to drink and feed. Many species of aquatic insects live only in fast-moving rivers and streams. Some fish, like American Eels, use rivers as a "highway" to and from breeding grounds.