Rivers carry water through landscapes and connect places that may look very different from one another. A narrow upland stream can eventually join a broad channel crossing a plain. Along the way, the flow receives water from smaller tributaries, rainfall, and groundwater. The boundaries of the surrounding drainage basin help explain why events far upstream can influence conditions much farther down the river.

A river channel is not a permanent line drawn on the ground. Moving water can erode material from one bank and deposit it elsewhere. Where a channel bends, differences in flow contribute to changes in its shape. Sandbars and gravel beds may grow, shrink, or shift after high water. These adjustments can happen slowly over many years or become especially noticeable during a single period of unusually strong flow.

The land beside a river is closely connected to the channel. Floodplains receive water when the river rises beyond its usual banks. This movement can carry fine sediment onto the surrounding ground. Plants growing along the banks provide shade and create shelter for animals. Their roots also interact with the soil, although the stability of any particular bank depends on its materials, slope, vegetation, and exposure to flowing water.

Rivers support many forms of movement besides the water itself. Fish and other aquatic organisms use different parts of the channel for feeding or shelter. Leaves, branches, dissolved substances, and small particles travel downstream. People have also used rivers as routes, sources of water, and places around which settlements develop. Bridges and embankments reveal how human activity responds to the opportunities and constraints of a changing channel.

Useful river observations include the height of the water, its clarity, the shape of the banks, and the materials lying on exposed bars. A river after heavy rain may look very different from the same reach during a dry spell. Comparing such conditions helps distinguish ordinary seasonal variation from longer changes. Seeing the channel as part of a whole basin gives a more complete picture than examining one short stretch in isolation.