Lower Columbia River Flood Stage-Frequency Study

The Corps of Engineers recently completed a study with updated flood profiles and peak stage-frequency curves on the Lower Columbia River. These products show the chance of river levels rising above an elevation of interest in any given year. The study area extends from Bonneville Dam to Astoria, Oregon, over 130 river miles on the Columbia River. The last time a comprehensive study like this was performed was 1991. The study included both natural conditions with no reservoirs present (unregulated), as well as the condition with all current reservoirs present (regulated). This study used current USACE risk-based approaches to update peak stage-frequency curves with estimates of uncertainty. This study analyzed three flooding modes separately: spring snowmelt, winter atmospheric river, and winter coastal. These flooding modes were then combined to produce results on an annual basis. This study included the effect of likely levee breaches throughout the system in the adopted stage-frequency curves. The study reflects current river hydraulics and climate conditions. Flood risk is not a constant through time—it changes as a result of both natural and human-induced causes. While flood risk has changed in the past and will likely continue to change in the future, this report represents a best estimate of current-day conditions. This study included a sea level change assessment to show how far sea level change would propagate upstream during floods.

Key findings from this report are listed below: -Upstream reservoir construction and operation has significantly reduced the risk of spring snowmelt flooding. -Winter atmospheric river flooding currently poses the dominant risk for most of the Lower Columbia. -The uncertainty in the stage-frequency estimates is dominated by the limited period of record. -River hydraulics appear to have changed through time.