Bathymetry, topography, and sediment grain-size data from the Elwha River delta, Washington, July 2018

Two dams on the Elwha River, Washington State, USA trapped over 20 million m3 of sediment, reducing downstream sediment fluxes and contributing to erosion of the river's coastal delta. The removal of the Elwha and Glines Canyon dams between 2011 and 2014 induced massive increases in river sediment supply and provided an unprecedented opportunity to examine the response of a delta system to changes in sediment supply. The U.S. Geological Survey (USGS) developed an integrated research program aimed at understanding the ecosystem responses following dam removal. The research program included repeated surveys of beach topography, nearshore bathymetry, and surface sediment grain size to quantify changes in delta morphology and texture following the dam removals. This USGS data release presents data collected during surveys of nearshore bathymetry, beach topography and surface sediment grain size from the Elwha River delta, Washington. Survey operations were conducted between July 23 and July 26, 2018 (USGS Field Activity Number 2018-648-FA) by a team of scientists from the U.S. Geological Survey Pacific Coastal and Marine Science Center (PCMSC), Washington State Department of Ecology (WA DOE), and Washington Sea Grant. Nearshore bathymetry data were collected using two personal watercraft (PWCs) and a kayak, each equipped with single-beam echosounders and survey-grade global navigation satellite systems (GNSS). Topography data were collected on foot with GNSS mounted on backpacks. Positions of the survey platforms were referenced to a GNSS base station placed on a nearby benchmark with known horizontal and vertical coordinates. Depths from the echosounders were computed using sound velocity profiles measured with a conductivity-temperature-depth (CTD) sensor during the survey. A total of 164 km of nearshore bathymetric survey lines and 212 km of topographic survey lines were collected during the 4 days of survey operations. Environmental conditions were favorable resulting in excellent coverage of the beach and nearshore region. Surface sediment was sampled using a small ponar, or 'grab', sampler on July 23, 2018 from the R/V Frontier at a total of 39 locations in water depths between about 1 and 17 m around the delta. An additional 35 sediment samples were collected between July 24 and July 26, 2018 at low tide from intertidal locations on the delta. A handheld GNSS receiver was used to determine the locations of sediment samples. Figure 1 (top) shows the locations of bathymetric- and topographic-survey lines, sediment samples, CTD samples, and GNSS base stations. A continuous DEM surface of the primary survey area was produced from all available bathymetry and topography data using linear interpolation and a grid-spacing of 5 m. Figure 1 (bottom) shows the bathymetry and topography data for areas adjacent to the river mouth color-coded by elevation. An additional DEM with 1-m resolution grid-spacing was produced using linear interpolation for this smaller region. Digital files containing the nearshore bathymetry data, beach topography data, derived DEMs, and grain-size data from this survey are available for download from the child item pages listed below. Figure 1. Top: Map showing locations of bathymetric- and topographic-survey lines, sediment samples, CTD casts, and GNSS base stations from the July 2018 survey of the Elwha River delta, Washington. Bottom: Zoomed in detail of top map showing bathymetric- and topographic-survey lines color-coded by elevation.