Channel Evolution Model

The changes a channel undergoes to restore equilibrium following a major disturbance such as increased urbanization within the watershed generally follow a recognizable pattern. These stages have been summarized in the channel evolution model. The ability to recognize a stream channel's stage in the channel evolution model has great utility for selecting appropriate restoration and management activities. Several different models of channel evolution exist; generally, five or six stages are recognized. The following five-stage model is attributed to Schumm et al. (1984) and Simon (1989). All images courtesy of the City of Austin, Texas.

Stage I: Stable

Stage I represents a stable stream channel before modification. In this stage, erosion and deposition are essentially equal within a stream reach; erosion occurs on the outside of stream bends and deposition occurs on the inside of bends. The stream bank height is lower than the height at which the banks have the potential to collapse (critical bank height). This is depicted as h < hc (height of banks is less than the critical bank height). The stream banks support vegetation.

Stage II: Constructed

A stream moves from a stable state into Stage II of the CEM following an increase in stream flow (Q) or stream slope (S; both variables are on right side of Lane's equation). In response, the stream must increase the discharge of sediment (Qs) or particle size (D50; left side of Lane's equation). Greater sediment discharge leads to downcutting of the streambed. Eventually, the height of the stream banks becomes higher than the critical bank height (h > hc). This stage is characterized by degradation (loss of material).

Stage III: Widening

In this stage, the banks which became higher than the critical height due to incision begin to collapse. As the banks collapse, the stream channel becomes progressively wider. In this stage, stream banks are still higher than the critical bank height. This stage is also characterized by degradation.

Stage IV: Stabilizing

The excessive erosion and bank collapse that occurred in Stage III provides an surplus of sediment which cannot all be removed by the stream. Through this accumulation of sediment, weak riffle-pool bed features begin to form. At the same time, the stream bank height begins to stabilize to approximately the critical height. The stream channel may shift among different channels within the main channel. Vegetation becomes established on the newly created banks. This stage is characterized by aggradation (accumulation of material).

Stage V: Stable

The channel evolution is complete and the channel is again stable, although at a lower elevation. Banks are lower than the critical bank height. Terraces may be visible; these are remnants of the original floodplain.

Streams can lose equilibrium as a result of accelerated sediment supply, accelerated bank erosion rates, degradation, aggradation from channel disturbance, streamflow changes, and sediment budget changes. These shifts in the components of Lane's equation are often the result of changes in land use including timber cutting, land clearing for agricultural purposes, road construction, and overgrazing. The loss of vegetation cover and soil compaction that often accompany these changes in land use can have visible impacts on stream channel morphology and function.