Spatial Distribution of Impacts to Channel Bed Mobility Due to Flow Regulation, Kootenai River, Usa

The regulated hydrograph of the Kootenai River between Libby Dam and Kootenay Lake has altered the natural flow regime, resulting in a significant decrease in maximum flows (60% net reduction in median 1-day annual maximum, and 77%-84% net reductions in median monthly flows for the historic peak flow months of May and June, respectively). Other key hydrologic characteristics have also been affected, such as the timing of annual extremes, and the frequency and duration of flow pulses. Moreover, Libby Dam has impeded downstream delivery of sediment from the upper 23,300 km of a 50,000 km watershed. Since completion of the facility in 1974, observed impacts to downstream channel bed and bars in semi-confined and confined reaches of the Kootenai River have included coarsening of the active channel bed, homogenization of the channel bed, disappearance of relatively fine-grained beach bars, and invasion of bar surfaces by perennial vegetative species. These impacts have led to reduced aquatic habitat heterogeneity and reduced abundance of candidate recruitment sites for riparian tree species such as black cottonwood (Populus trichocarpa) and native willows (Salix spp.). Limited quantitative documentation of the pre-regulation substrate composition exists. However, based on the existing evidence assembled by others, we have assumed that the contemporary river bed is relatively coarser than the pre-regulation bed. Making this assumption, we tested the hypothesis that even though the contemporary bed is relatively coarser than the historic bed condition, the pre-regulation hydrograph applied to the contemporary bed would result in ‘regular’ bed movement in most sections of the study reach, while the regulated hydrograph provides limited opportunities for bed and bar adjustment. Our sampling effort to document the contemporary bed composition over a 100-kilometer channel section included stratification of the study reach by reach-scale characteristics, and underwater photography-based methods. We present the results in a spatially-distributed manner that allows evaluation of both local factors and downstream trends in hydrologic alteration through the study reach.