A basic characteristic of streamflow that distinguishes it from sheet flow is that it is usually confined to a channel. A stream channel can be thought of as an open conduit that consists of the streambed and banks that act to confine the flow, except during floods. Although somewhat oversimplified, we can divide stream channels into two types. Bedrock channels are those in which the streams are actively cutting into solid rock. In contrast, when the bed and banks are composed mainly of unconsolidated sediment, the channel is called an alluvial channel.