Long-Term Evolution of Columbia River Tides

A 170 km tidal channel connects the lower Columbia River ports of Portland and Vancouver to the Pacific Ocean. An increasing incidence of low water levels during summer and fall sometimes limits the draft of ships using the channel and can delay transits. Reduced spring freshet water levels have also decreased shallow-water habitat for juvenile salmon. Records from Tongue Point (km-30, 1925-date), and Vancouver (km-175, 1902-date) were used to analyze historic changes. Mean water levels (MWL) at Vancouver have dropped ∼0.3–1.5 m since 1902, depending on flow. Lower low water (LLW) at the lowest river flows has decreased ∼0.4–0.5 m since 1940. Vancouver LLW has dropped more than MWL and higher high water (HHW), increasing tidal range. Tides in the Columbia and many other river ports are evolving because of a combination of changing ocean tides and local alterations, including both harbor development and river-flow modification. Assessments of climate change impacts on harbors need to consider both factors.