When it rains on Earth, some water soaks into the ground and then reappears in springs. Most upland streams get their water directly from springs.

Streams that are fed by springs often flow down steep slopes.

As the headwaters of a stream rush through rapids and waterfalls, they trap many air bubbles which contain oxygen. Oxygen from these air bubbles can easily dissolve in the water.

Thus, streams are usually well-oxygenated at their headwaters.

There are usually no plankton - drifting organisms that are often microscopic - in the turbulent headwaters of a stream.

Green algae, diatoms and water mosses may be attached to stones or other objects in the water. These organisms may completely cover the bottom of the stream.

These organisms are eaten by insect larvae, which are themselves eaten by small fish.

Insects continually fall into the stream, and rain washes detritus (dead organic matter) into it as well.

Whatever is not eaten immediately washes further downward, so there is very little food for detritivores (also known as saprovores or saprophages), organisms that eat detritus, in the headwaters.

As the water moves on, it begins to move more slowly.

The bed of the stream becomes larger, and the total volume of water increases.

Some of the sediment from upstream is deposited.

Dead organic matter accumulates, so there is now food for detritivores to consume.

As the stream grows wider and starts to become a river, less water is shaded by the trees belong the bank. This means that direct sunlight can reach most of the surface of the water.

As the level of light increases, the rate of photosynthesis increases.

There are some plankton, but many plankton organisms are swept downstream.

Plants with roots grow in the sediments at the bottom. During floods, these plants may be washed away.

Snails, mussels, crayfish and insect larvae live at the bottom.

These may be eaten by perch and trout.

Leeches may feed on these fish.

As a river comes close to the sea, it usually slows down even further. It drops large amounts of sediment.

Banks along the lower reaches of the river may grow higher than the land behind it, forming natural levees.

Waters in this part of a river are usually muddy.

Because of this, the amount of sunlight that can penetrate the water is reduced, and organisms that depend on photosynthesis cannot live at the bottom.

However, many plankton - some of which perform photosynthesis - live in the waters near the surface.

Floating plants and emergent plants - plants that extend from below the water into the air - grow in the swampy lands along a river.

During floods, the fruits and seeds of these plants are swept into the river.

Large predators at the lower end of a river eat zooplankton - animal forms of plankton.

Large fish, large crustaceans and large mollusks live at this end of the river.

Many birds and mammals come here to obtain food.

Crocodiles can be found at the lower ends of tropical rivers.