EROSION PATTERNS IN ELKHORN SLOUGH, CENTRAL CALIFORNIA

Abstract:

The Elkhorn Slough in central California has gone through periods of erosion and high energy currents in the 1870s and periods of more deposition and lower energy in the 1920s-1940s. Since the construction of jetties at Moss Landing Harbor in the 1940s, the harbor mouth has dramatically increased tidal currents and with them, the patterns of erosion. Current strong tidal incursion (particularly ebb tides) has changed this environment into a very erosive one, transforming the mud banks and channel floor quickly. In order to evaluate the erosion processes in the Slough’s underwater sediments we conducted two seismic surveys in 2012 and 2014. The EdgeTech SB-424 full-spectrum sub-bottom CHIRP profiler, was used with a default pulse which generates a sweep frequency of 4 kHz – 24 kHz for 10 ms, and a vertical resolution of 0.4 m. This system performed ideally in this shallow environment.

The chirp lines show channels that abrade layers of older sediment, and areas with a very highly reflective seafloor reflector that we interpret as coarse sediments. This erosive process is extremely pronounced near the ocean (in the western end) and in the middle of the channel. Comparison of the sedimentation at the mouth and farther inland suggests that the velocity of water is significantly greater at the mouth. We discuss the patterns of sedimentation/erosion in the mouth of the slough and away from the ocean. We also discuss the differences in two years’ time and if there is any evidence of changes in the geological processes. If current patterns continue, more and more of the slough will be weathered away, thus affecting the numerous species that currently live in it.

Funding provided by the Office of Naval Research.