Response to disturbance in a highly managed alluvial river: Does it conform to Le Chatelier's general law?

A magnitude 6.8 earthquake triggered a channel-damming landslide causing the lower Cedar River, Washington, to occupy an existing floodplain side channel. The channel and floodplain ecosystem initially underwent rapid geomorphic changes until the system energy approached a new system state according to Le Chatelier's general law. Multiple lines of evidence support observations that channel expansion proceeded rapidly as energy was focused on the banks of the overfit side channel until the channel's geomorphology was consistent with total system energy available for work. Large trees recruited to the channel helped create lateral channel expansion as well as bed topographic variability. One year of pre-disturbance data was compared with three years of post-disturbance data in a spatially explicit analysis of the complexity of channel form and bed surface elevations. Estimates of changes in sediment storage and erosion and, channel conveyance capacity were calculated for all time periods and interannual comparisons were carried out. Statistical analyses of aggraded and eroded areas of the channel indicated a rapid increase in complexity of the channel during the first three years and then exhibited decreasing complexity in the final year of the study.

► We compared one year of pre- with three years of post-disturbance data.
► We estimated aggraded and eroded sediment volumes using DEM differencing.
► We quantified interannual channel heterogeneity using fractal analysis and ANCOVA.
► Channel expansion was dramatic but conveyance capacity remained fairly constant.
► There are indications that “dynamic equilibrium” returned within three years.