The Transboundary Reach of the Columbia River: Cottonwood Colonization Followed Flow Moderation From the Columbia River Treaty Dams

ABSTRACT The Columbia river provides the largest Pacific outflow in the Western Hemisphere and the greatest hydropower production of any North American river system. For hydropower generation and flood risk management, four massive water storage reservoirs followed the Columbia River Treaty between Canada and the United States, with three Canadian dams, Mica, Keenleyside, and Duncan, and Libby Dam in the USA. The transboundary reach (TBR) of the Columbia river is the final river segment in Canada and downstream from all four dams. The dams are operated to attenuate spring peak flows for flood risk management, and to augment winter flows for hydropower generation. This provided seasonal flow moderation, and black cottonwoods subsequently colonized gravel bars and islands along the TBR that were previously barren due to frequent and prolonged inundation. The cottonwood groves contribute to the river's aquatic food web and provide rich wildlife habitats and other ecosystem services. The cottonwoods colonized surfaces above the river margins with flows of ~3200 m 3 /s. This elevation experienced an annual average of 23 days of inundation and > 40 inundation days in 1‐in‐4‐years. These values provide inundation tolerance thresholds for cottonwood colonization in riverine riparian zones or along upper drawdown zones at storage reservoirs. For this analysis we derived a 112‐year flow record for the TBR Columbia, which displayed no overall temporal trend in mean annual discharge. Multiple‐year intervals with lower or higher river flows were correlated with the pacific decadal oscillation through the twentieth century, but that teleconnection was apparently uncoupled after about 1990.