Surface-sediment grain-size distributions of the Elwha River delta, Washington, August 2019

Surface sediment was collected on August 26, 2019 at a total of 77 locations using a small ponar, or 'grab', sampler from the R/V Frontier in water depths between about 1 and 17 m around the delta. An additional 30 samples were collected by hand at low tide between August 27 and August 29, 2019. A hand-held global satellite navigation system (GNSS) receiver was used to determine the locations of sediment samples. Grab samples that yielded less than 50 g of sediment were omitted from analysis.

The grain-size distributions of suitable samples were determined in the USGS Pacific Coastal and Marine Science Center sediment lab. Approximately 20 g of sediment was sub-sampled and 10 mL of 35 percent hydrogen peroxide was added to remove organic material in the sample. The sample was heated at 250 degrees C to remove any remaining hydrogen peroxide and was placed in an ultrasonic bath to liberate the fine fraction. Subsequently, the sample was centrifuged for 1.5 hours to remove soluble salts. Samples were funneled through 2 mm and 63 um sieves to separate the gravel (greater than 2 mm), sand (63 um to 2 mm), and mud (less than 63 um). The sand and gravel fractions were dried in an 80-degree C oven and weighed. Five mL of sodium hexametaphosphate was added to the mud fraction and a 20 mL aliquot was taken from a 1 L graduated cylinder to determine the weight of the mud. The particle-size distribution of the mud and sand fractions was analyzed separately using a Beckman Coulter LS 13 320, and the gravel, sand, and mud results were integrated into a single grain-size distribution for each sample.