A basic characteristic of streamflow that distinguishes it from sheet flow is that streamflow is 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 this is somewhat oversimplified, we can divide stream channels into two types. A bedrock channel is one in which the stream is 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.

Bedrock Channels

As the name suggests, bedrock channels are cut into the underlying strata and typically form in the headwaters of river systems where streams have steep slopes. The energetic flow tends to transport coarse particles that actively abrade the bedrock channel. Potholes are often visible evidence of the erosional forces at work.

Steep bedrock channels often develop a sequence of steps and pools. Steps are steep segments where bedrock is exposed. These steep areas contain rapids or, occasionally, waterfalls. Pools are relatively flat segments where alluvium tends to accumulate.

The channel pattern exhibited by streams cutting into bedrock is controlled by the underlying geologic structure. Even when flowing over rather uniform bedrock, streams tend to exhibit winding or irregular patterns rather than flow in straight channels. Anyone who has gone whitewater rafting has observed the steep, winding nature of a stream flowing in a bedrock channel.

Alluvial Channels

Many stream channels are composed of loosely consolidated sediment (alluvium) and therefore can undergo significant changes in shape because the sediments are continually being eroded, transported, and redeposited. The major factors affecting the shapes of these channels are the average size of the sediment being transported, the channel gradient, and the discharge.

Alluvial channel patterns reflect a stream’s ability to transport its load at a uniform rate, while expending the least amount of energy. Thus, the size and type of sediment being carried help determine the nature of the stream channel. Two common types of alluvial channels are meandering channels and braided channels.

Meandering Streams

Streams that transport much of their load in suspension generally move in sweeping bends called meanders. These streams flow in relatively deep, smooth channels and transport mainly mud (silt and clay), sand, and occasionally fine gravel. The lower Mississippi River exhibits a channel of this type.

Meandering channels evolve over time as individual meanders migrate across the floodplain. Most of the erosion is focused at the outside of the meander, where velocity and turbulence are greatest. In time, the outside bank is undermined, especially during periods of high water. Because the outside of a meander is a zone of active erosion, it is often referred to as the cut bank (Figure 1). Debris acquired by the stream at the cut bank moves downstream, where the coarser material is generally deposited as point bars on the insides of meanders. In this manner, meanders migrate laterally by eroding the outsides of the bends and depositing sediment on the insides without appreciably changing their shape.

In addition to migrating laterally, the bends in a channel also migrate down the valley. This occurs because erosion is more effective on the downstream (downslope) side of the meander. Sometimes the downstream migration of a meander is slowed when it reaches a more resistant bank material. This allows the next meander upstream to gradually erode the material between the two meanders, as shown in Figure 2. Eventually, the river may erode through the narrow neck of land, forming a new, shorter channel segment called a cutoff. Because of its shape, the abandoned bend is called an oxbow lake.

Braided Streams

Some streams consist of a complex network of converging and diverging channels that thread their way among numerous small islands or gravel bars. Because these channels have an interwoven appearance, they are called braided channels. Braided channels form where a large proportion of the stream’s load consists of coarse material (sand and gravel) and the stream has a highly variable discharge. Because the bank material is readily erodible, braided channels are wide and shallow.

One setting in which braided streams form is at the end of glaciers, where there is a large seasonal variation in discharge (Figure 3). During the summer, large amounts of ice-eroded sediment are dumped into the meltwater streams flowing away from the glacier. However, when flow is sluggish, the stream deposits the coarsest material as elongated structures called bars. The flow then splits into several paths around the bars. During the next period of high flow, the laterally shifting channels erode and redeposit much of this coarse sediment, thereby transforming the entire streambed. In some braided streams, the bars have built semipermanent islands anchored by vegetation.

• Bedrock channels are cut into solid rock and are most common in headwater areas where gradients are steep. Rapids and waterfalls are common features.

• Alluvial channels are dominated by streamflow through alluvium previously deposited by the stream. A floodplain usually covers the valley floor, with the river meandering or moving through braided channels.

• Meanders change shape through erosion at the cut bank (the outer edge of the meander) and deposition of sediment on point bars (the inside of a meander). A meander may become cut off and form an oxbow lake.

• braided channels: Stream channels consisting of numerous intertwining channels separated by bars of sand and gravel.

• cut bank: The area of active erosion on the outside of a meander.

• cutoff: A short channel segment created when a river erodes through the narrow neck of land between meanders.

• meanders: Looplike bends in the course of a stream.

• oxbow lake: A curved lake produced when a stream cuts off a meander.

• point bars: Crescent-shaped accumulations of sand and gravel deposited on the inside of a meander.