Classifying Stream Equilibrium

Stream equilibrium is a method for classifying the status of a system between Aggradation and Degradation, which extends beyond the standard range of Erosion and Deposition of sediment.

Lane's scale equating the product of sediment load and sediment size with the product of Stream slope and Stream discharge is shown in this figure, modified by Rosgen. The dynamic equilibrium of natural Erosion and Deposition is upset when one of these variables shifts excessively. For example, if Stream slope increases due to removal of Meander bends and straightening of the channel, then Degradation will likely occur. Eventually, sediment load increases to the degree that the balance may reestablish, or given the sensitivity of the system, swing back too far and into Aggradation as that extra sediment settles out downstream. This process parallels the explanation for the channel evolution model.

Image courtesy of Rosgen