Stream channel stability is often misunderstood. A stable channel is not necessarily static, and stable channels frequently move. An indication of a stable channel can be that it moves laterally across its floodplain but maintains its channel dimensions. Stable channels maintain over time, a balance between erosion and deposition. They neither degrade (downcut) or aggrade (fill in). The concept is not new. Hoover Mackin (1948) introduced the term “graded river” to describe a fluvial system that had developed a geometry, pattern, and slope that allowed for the efficient transport of the water, debris, and sediment loads within the channel, but retained no additional energy for erosion. This concept recognizes that the dimensions of natural channels continually adjust in order to achieve equilibrium with regard to water inputs and subsequent transport of sediment and debris.
Lane (1955) depicted the equilibrium relationship with an image of a balance (see image below). On the left side of the image, sediment ranges from coarse to fine. As median particle size (D50) and relative total sediment load (Qs) increase, so too does the amount of energy required to transport the total sediment load. Inversely, on the right side of the image, as water gains energy with increases in slope (S) and relative load of water (Qw), more work, in the form of sediment transport, can be achieved.
Channel instability is characterized by an imbalance between erosive and depositional forces. Once a system falls outside a state of equilibrium, either with increased inputs of sediment or water, they can be described as aggrading (building up with increased sediment) or degrading (increased erosion and loss of channel features). Rates of channel adjustments and overall channel stability can be the result of natural or anthropogenic changes. For example, when a “stable” channel has been subjected to an increase in flow as a result of deforestation, upstream channel modification, or an increase in the area of impervious surfaces within the watershed, the channel will respond in a way as to return to equilibrium. In this case, that equilibrium must come from an increased sediment load that allows channel-building processes to resume. Unfortunately, these sediments generally source from a combination of erosive processes than can include channel incision, widening, and ultimately bank failure.
Adjustments made by stable channels tend to occur within the area known as the channel corridor. This “corridor” is defined as the combined area of the channel, its banks, and the adjacent valley bottom. Defining the corridor that may be effected by channel adjustments is the goal of fluvial hazard mitigation mapping. Please see the mapping section of this website for examples.
Lane, E. W., 1955. The importance of fluvial morphology in hydraulic engineering. Proceedings, American Society of Civil Engineers, Vol. 81, Paper 745, July.
Mackin, J.H., 1948. Concept of the Graded River. Geologic Society of America Bulletin 1948;59, no. 5;463-512