Rivers and streams generally support communities of organisms quite different from those of lakes and ponds. A river or stream changes greatly between its source and the point at which it empties into a lake or the sea. Near the source, a stream’s water is usually cold, low in nutrients, and clear. The channel is often narrow, with a swift current that does not allow much silt to accumulate on the bottom. Most of the organisms found here are supported by the photosynthesis of algae attached to rocks or by organic material, such as leaves, carried into the stream from the surrounding land. Downstream, a river or stream generally widens and slows. The water is usually warmer and may be cloudier because of sediments and other particles suspended in it. Worms and insects that burrow into the mud are abundant, as are waterfowl, frogs, fish, and other water animals.