Grain size on the Elwha River delta intertidal beach: impacts of dam removal and implications for Pacific sand lance (Ammodytes hexapterus)

Abstract

Small mountainous rivers, such as the Elwha River in Washington State, play a key role in the sedimentary processes that form and maintain the structure and function of their neighboring shorelines. The Elwha River was dammed from 1910 to 2011, and construction of the dams has been attributed to coarsening and erosion of its delta and adjacent shorelines. In turn, the habitat made available to marine organisms was altered for nearly 100 years. This study examined the median (D50) of the grain sizes, sand fractions, and respective elevations of physical samples from the Elwha delta using dry and wet sieve methods, as well as average surface grain sizes using photographic methods, to evaluate the impacts of dam removal of the restoration and/or expansion of Pacific sand lance (sand lance) spawning habitat. Of the 34 physical samples collected and as of 12 and 13 April 2014, the findings suggest 10 sample locations west and 8 samples locations east of the river mouth met the three parameters documented as preferred sand lance spawning habitat. Each of the 18 locations within the study area had grain-size distributions with medians ranging between ~0.2 and 2.0 mm, the bulk sand fractions of each sample ranged between ~60 and 98%, and their respective elevations were between +1.33 and +2.22 m MLLW. Potentially suitable spawning habitat for sand lance was found ~1,230 m west and ~1,500 m east of the river mouth.