Response to disturbance in a highly managed alluvial river: Does it conform to Le Chatelierʹs 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 Chatelierʹs 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 channelʹs 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.