Sea Level, Tidal, and River Flow Trends in the Lower Columbia River Estuary, 1853–present

Few tidal records are available pre‐1900 for the Pacific Ocean. We improve data coverage by recovering historical tabulations and digitizing analog tide rolls from Astoria, Oregon, for 1853–1876. Nearly 13,500 overlapping images of tides from 1855–1870 were digitized at a 6 min resolution using a line‐finding algorithm. Available hourly and high/low tabulations were also digitized, as were nearby hourly records from 1933 to 1943. Uncertainty was assessed by evaluating manual staff measurements, historical documents, and leveling surveys. Results suggest that uncertainty in mean sea level varies from ±0.07 m (early 1850s) to ±0.03 m (1867–1876) and is driven primarily by datum and benchmark uncertainty, rather than measurement precision, data reduction procedures, or hydrodynamic changes. We also corrected an up‐to 0.05 m error in the 1925–1960 tidal datum at Astoria. Harmonic analysis shows that major tidal constituents increased by up to 7% between 1855 and 2018. Mean tidal range increased by 0.1 m (5%), with more change occurring in July (0.17 m larger) than winter (0.07 m larger). By contrast, sea level increased most in winter and least in spring/summer. Tidally based estimates of river discharge suggest that these observations are caused by a ~50% reduction in peak spring discharge and a 30–60% increase in winter discharge. No evidence of altered upwelling is found. Overall, Astoria relative sea level (RSL) increased by 0.06 m ± 0.04 m since the 1858–1876 epoch or, after accounting for vertical land motion, 0.11 ± 0.09 m. Consistent with GNSS measurements, RSL has dropped near the estuary mouth since 1905, indicating a strong tectonic influence. Key Points Extensive data archeology and digitization of analog records have been used to produce high‐resolution data from 1853 to 1876 at Astoria Relative sea level rise since the 1850s is 0.06 m; we estimate a geocentric rise of 0.11 m, after accounting for vertical land motion Significant variations in seasonal sea level and tides are observed due to altered river flow; a secular increase of 0.1 m in tidal range is observed