HUMAN-INDUCED COASTAL EVOLUTION OF THE COLUMBIA RIVER LITTORAL CELL

Examination of the historical coastal evolution of the Columbia River littoral cell in the U.S. Pacific Northwest reveals that human interventions have profoundly altered the patterns and trends of bathymetric and shoreline change. While the construction of dams throughout the drainage basin has reduced fluvial supply of sand to the estuary, the construction of jetties at the entrances to the Columbia River (1885–1917) and Grays Harbor (1898–1916) has accentuated the morphological coupling between the inlets, ebb-tidal deltas, shorefaces, and barriers. Prior to jetty construction, the adjacent coasts were co-linear and linked across the entrance by shallow ebb-tidal shoals and shore-connected platforms that were no more than 5 m deep. With jetty construction and dredging, the adjacent coasts became offset to the north by roughly 2 km, and the shallowest contiguous contour across the entrances is now more than 20 m deep. The jetties have induced erosion of the inlets and offshore migration of ebb-tidal deltas, causing the depth of the inner deltas to roughly double. The change in boundary conditions at these inlets has enabled waves to rework the flanks of ebb-tidal deltas and supply large quantities of sand to the adjacent coasts. Over several decades since the jetties were built, these inherited shoreface sand bodies have been a primary source of sand for progradation of adjacent beaches up to tens of kilometers from the estuary entrances. Long-term shoreline progradation rates across the littoral cell increased from roughly 1.4 m/yr during the late prehistoric period (1700–1860s) to roughly 10 m/yr during the initial decades following jetty construction. Averaged over the entire historical period since jetty construction, the shoreline progradation rates are approximately double the pre-jetty shoreline progradation rates. Until recent decades, sediment supply from the ebb-tidal delta flanks and lower shoreface has largely masked the decline in Columbia River sediment supply due to flow regulation and dredging disposal practices. With the contemporary onset and expansion of coastal erosion adjacent to the jettied estuary entrances, strategic management of dredged material is imperative to mitigate the effects of a declining sediment budget.