LATERAL MOBILITY OF RIVER CHANNELS: MEASUREMENT AND EXTERNAL CONTROLS

We analyze area-based change in alluvial river channel plan form over time in order to quantify channel mobility with respect to given external parameters. Our studies of channel migration use both field and experimental data to define average rates of channel migration with respect to the ratio of sediment flux to channel bankfull height (as a characteristic vertical length-scale). We then combine this value with an avulsion timescale (given by superelevation in depositional settings) to develop a simple mathematical expression for general channel mobility.

In order to look at channel mobility and self-memory holistically, we analyze five analog experiments, performed between 2002 and 2007 at the Saint Anthony Falls Laboratory. These experiments isolate the effects of (1) sediment input and deposition, (2) base-level, and (3) bank cohesion on channel mobility. Our methods show that the similarity between the channel plan form at the initial time-step and the channel plan form at every consecutive step decays exponentially with time. (1) In experimental deltaic environments in which aggradation and sea-level rise were in equilibrium, the lower-aggradation case had higher-than-expected channel mobility due to steeper-than-expected deltaic slopes and the presence of channel-depth-scale ripples. (2) Channels in an experimental basin with rapid (12.2 mm/hr) base-level change are three times as mobile during base-level rise as during base-level fall. Channels in a cohesive delta experiment (producing significantly lower channel mobility) are 1.5 times as mobile during slow base-level rise (0.237 mm/hr) as during steady base level. Base-level change that is slow relative to a channel mobility timescale has little to no influence on channel mobility. (3) Channels in an experimental system of noncohesive sediment are 6.5 times as mobile as channels with vegetation (alfalfa) planted in the flume.