The erosion and transport of rock and sediment by a stream defines the shape and extent of its
valley. V‐shaped valleys and wide valleys with flat floors are the most common varieties.
Downcutting. A valley is the result of downcutting, whereby a stream's channel erodes directly downward. As downcutting continues, erosion and mass wasting begin to work on the exposed, vertical sides of the channel, eroding them into slopes and widening the valley (Figure 1).
A Downcutting Stream Profile
Slot canyons, vertical‐walled rock canyons where mass‐wasting processes have been very limited, are a common feature in the western United States (Figure 2).
As a stream flows downslope and gains more water from tributaries, the valley becomes wider because of greater mass wasting. Downcutting proceeds until the base level is reached—the elevation of the most horizontal flow and lowest velocity. For streams that empty into the ocean, base level is essentially sea level. Base level for continental streams is generally the lowest elevation of the valley.
Ungraded and graded streams. An ungraded stream is one that is still actively downcutting and smoothing out its irregular gradient through erosion. It is characterized by rapids and waterfalls. In contrast, a graded stream has smoothed out its longitudinal profile to resemble a smooth, concave‐upward curve (Figure 3). How long it takes a stream to become graded is influenced by sediment load, which affects the rates of erosion and downcutting. Dams also have a major impact on grading by reducing stream flow and sediment load.
An Ungraded and a Graded Stream
Headward and lateral erosion. Valleys are further developed by headward erosion and lateral erosion. Headward erosion results when a valley is extended upward above its original source by gullying, mass wasting, and sheetwash flow. Lateral erosion occurs when the stream meanders or braids back and forth across its valley floor or channel, undercutting and eroding its banks. This results in mass wasting of the gradually more unstable slopes and forms a wider floodplain.
Stream terraces. Stream terraces are steplike benches that occur above the stream bed and floodplain. They are cut into bedrock or are remnants of older river sediments that have since been eroded. Terraces form in response to flooding or changes in base level.
Incised meanders. Incised meanders are steep‐walled canyons that result from the downcutting of a meandering stream. Usually without floodplains, they are thought to be the result of the uplift of a meandering stream above its base level.