An Evaluation of Sediment Transport in the Columbia River Basin: Before and After System-Wide Flow Regulation

Erosion and sediment transport are ubiquitous in every riverine ecosystem. Aquatic habitat eroded and sediment removal for infrastructure require deposition of upstream material to maintain morphodynamic equilibrium. A large percentage of total sediment transport occurs during relatively brief high flow events throughout the water-year. Dams are known to modify the ability for streams to freely transport sediment. Depending on the river size and dam type, downstream movement of sediment can be inhibited or completely eliminated. On the Columbia River, 14 dams are situated between the tidal-zone to the headwaters in the Canadian Rocky Mountains. The main uses of the dams are navigation, flood control, and power generation. A majority of the dams are “run-of-the-river” style installations without long-term storage capacity, because of high river flow and reservoir geometry. Run-of-the-river dams generally allow some sediment transport to occur even of the coarser grain sizes. In 1970, the final dams were almost completed on the mainstem Columbia River and lower Snake River, this date also marks the beginning of complete system-wide flow regulation (SWFR). Sediment transport has been reduced by the implementation of SWFR, and not solely because dams are in place along the river. This study estimates sediment transport before and after 1970 to evaluate possible changes after complete SWFR began. Pre-SWFR data from 1962-63 and 1968-69 have been digitized for 4 sites along the mainstem Columbia River and near the confluence of two of its largest tributaries. Post-SWFR sites were selected as close as possible to pre-SWFR data collection areas and were obtained from the US Geological Survey (USGS) and the US Army Corps of Engineers (USACE). Pre- and post-SWFR models were created using water-discharge as an explanatory variable to hindcast and forecast total, fine, and coarse sediment transport for the period of 1887 to 2023 for Warrendale, Willamette and Beaver sites, 1933 to 2023 for the Pasco site, 1958 to 2023 for the Snake River site, and 1960 to 2023 for the Vancouver site. Reconstruction of the entire period of record at each site is used to validate each model. Additional model validation occurred on the Lower Columbia River and the Willamette River with recently installed acoustic-derived sediment gages. Post-SWFR models match well with acoustic sediment predictions, but these modern observing systems have not recorded data during extreme events. Generally, models perform best using coarse sediment loads and show large decreases in sediment transport post-SWFR. Pre-SWFR models have R2 values ranging from 0.60 to 0.99; the best model fits are associated with the coarse sediment transport data. Post-SWFR models have R2 values ranging from 0.48 to 0.98 with coarse sediment transport again having the best fits. Hindcast sediment discharge daily values were summed over each water year and were used to evaluated changes in sediment transport over the entire available water discharge period. Sediment transport at mainstem Columbia River sites has declined 49% to 65% for total sediment transport, 31% to 67% for fine sediment transport, and 72% to 88% for coarse sediment transport.