A stream valley consists of a channel and the surrounding terrain that directs water to the stream. It includes the valley floor, which is the lower, flatter area that is partially or totally occupied by the stream channel, and the sloping valley walls that rise above the valley floor on both sides. Alluvial channels often flow in valleys that have wide valley floors consisting of sand and gravel deposited in the channel and clay and silt deposited by floods. Bedrock channels, on the other hand, tend to be located in narrow V-shaped valleys. In some arid regions, where weathering is slow and rock is particularly resistant, narrow valleys having nearly vertical walls are also found. Such features are called slot canyons. Stream valleys exist on a continuum from narrow, steep-sided valleys to valleys that are so flat and wide that the valley walls are not discernible.

Streams, with the aid of weathering and mass movement, shape the landscape through which they flow. As a result, streams continuously modify the valleys they occupy.

Base Level and Stream Erosion

Streams cannot endlessly erode their channels deeper and deeper. There is a lower limit to how deep a stream can erode, a limit called base level. Although the idea is relatively straightforward, it is nevertheless a key concept in the study of stream activity. Base level is defined as the lowest elevation to which a stream can erode its channel. Essentially this is the level at which the mouth of a stream enters the ocean, a lake, or another stream. Base level accounts for the fact that most stream profiles have low gradients near their mouths because the streams are approaching the elevation below which they cannot erode their beds.

Two general types of base level are recognized. Sea level is considered the ultimate base level because it is the lowest level to which stream erosion could lower the land. Temporary, or local, base levels include lakes, resistant layers of rock, and main streams that act as base levels for their tributaries. For example, when a stream enters a lake, its velocity quickly approaches zero, and its ability to erode ceases. Thus, the lake prevents the stream from eroding below its level at any point upstream from the lake. However, because the outlet of the lake can cut downward and drain the lake, the lake is only a temporary hindrance to the stream’s ability to downcut its channel. In a similar manner, the layer of resistant rock at the lip of a waterfall acts as a temporary base level. Until the ledge of hard rock is eliminated, it limits the amount of downcutting upstream.

Any change in base level causes a corresponding readjustment of stream activities. When a dam is built along a stream, the reservoir that forms behind it raises the base level of the stream (Figure 1). Upstream from the dam the gradient is reduced, lowering the stream’s velocity and, hence, its sediment-transporting ability. The stream, now having too little energy to transport its entire load, deposits sediment that builds up its channel. Deposition is the dominant process until the stream’s gradient increases sufficiently to transport its load.

Valley Deepening

When a stream’s gradient is steep and the channel is well above base level, downcutting is the dominant activity. Abrasion caused by bed load sliding and rolling along the bottom, and the hydraulic power of fast-moving water slowly lower the streambed. The result is usually a V-shaped valley with steep sides. A classic example of a V-shaped valley is the section of the Yellowstone River shown in Figure 2.

The most prominent features of a V-shaped valley are rapids and waterfalls. Both occur where the stream’s gradient increases significantly, a situation usually caused by variations in the erodibility of the bedrock into which a stream channel is cutting. Resistant beds create rapids by acting as a temporary base level upstream while allowing downcutting to continue downstream. In time, erosion usually eliminates the resistant rock. Waterfalls are places where the stream makes an abrupt vertical drop.

Valley Widening

Once a stream has cut its channel closer to base level, downward erosion becomes less dominant. At this point, the stream’s channel takes on a meandering pattern, and more of the stream’s energy is directed from side to side. The result is a widening of the valley as the river cuts away first at one bank and then the other (Figure 3). The continuous lateral erosion caused by shifting of the stream’s meanders produces an increasingly broad, flat valley floor covered with alluvium. This feature, called a floodplain, is appropriately named because when a river overflows its banks during a flood stage, it inundates the floodplain.

Over time the floodplain widens to the point that the stream is actively eroding the valley walls in only a few places. In fact, with large rivers, such as the Mississippi River, the distance from one valley wall to another can exceed 160 kilometers (100 miles).

Changing Base Level and Incised Meanders

We usually expect a stream with a highly meandering course to be on a floodplain in a wide valley. However, certain rivers exhibit meandering channels that flow in steep, narrow valleys. Such meanders are called incised meanders (incisum = to cut into).

How do these features form? Originally the meanders probably developed on the floodplain of a stream that was in balance with its base level (Figure 4A). Then, a change in base level caused the stream to begin downcutting. Such a change can be caused either by a drop in a downstream base level or by uplift of the land on which the river is flowing. For example, regional uplifting of the Colorado Plateau in the southwestern United States generated incised meanders on several rivers (Figure 4B). As the plateau gradually rose, meandering rivers began downcutting because of their steepening gradient.

Other features associated with a relative drop in base level are stream terraces. After a river that had been flowing on a floodplain has adjusted to a relative drop in base level, it may once again produce a floodplain at a level below the old one. As shown in Figure 5, the remnants of the former floodplain are present as relatively flat surfaces above the newly forming floodplain.

• A stream valley includes the channel itself, the adjacent floodplain, and the relatively steep valley walls. Streams erode downward until they approach base level, the lowest point to which a stream can erode its channel. A river flowing toward the ocean (the ultimate base level) may encounter several local base levels along its route. These could be lakes or resistant rock layers that retard downcutting by the stream.

• A stream valley is widened through the meandering action of the stream, which erodes the valley walls and widens the floodplain. If base level drops or if the land is uplifted, a stream downcuts. If it is underlain by bedrock, the stream may develop incised meanders. Streams underlain by deep alluvium are likely to develop terraces.

• base level: The lowest land level to which a stream can erode.

• floodplain: The flat, low-lying portion of a stream valley, adjacent to the stream channel, that is subject to periodic inundation.

• incised meaders: Meandering channels that flow in a steep, narrow valley. Incised meanders form either when an area is uplifted or when base level drops.

• stream terraces: Flat, benchlike structures left elevated as a stream cuts downward.

• stream valley: The channel, valley floor, and sloping valley walls of a stream.