The river continuum.

Most of the invertebrates of streams and rivers live on or in the sediments; that is, most are benthic. These benthic organisms are influenced substantially by the type of bottom sediments. Stony substratum in the riffles and runs of rivers harbour fauna and flora that are different from those in sections with silt or sand bottoms mainly because of differences in the structure and stability of these bottom types. River ecologists have recently discovered that a great number and diversity of invertebrate animals live deep within the sediments of rivers in both the hyporheic and phreatic zones. These species may be pumped up with well water many kilometers from the nearest river. We know very little about the lives of these organisms. Once more, nature has yielded another surprise.