Summary

The Flood Control Zone District for Yakima County, Washington, is currently (2009) studying levee setback and floodplain restoration on the Yakima and Naches Rivers. Part of the study includes the use of a sediment transport model that requires river bathymetry and near-shore topography. River-survey data collected after 2000 for most of the floodplain between Union Gap and Selah Gap were available prior to this project. However, data are lacking in or near Union Gap and Selah Gap, two topographic gaps near the City of Yakima, Washington. In August 2008, the U.S. Geological Survey surveyed the rivers in the areas that lacked bathymetric and topographic data. The data will be used by Yakima County and the Bureau of Reclamation to construct hydraulic and sediment-transport models of the river reaches to aid in floodplain management. The river was surveyed using a boat-mounted, survey-grade, single-beam echo sounder attached to a survey-grade RTK GPS. A walking survey with an RTK GPS rover was used for river banks, shallow river areas, and river-bed areas that were impenetrable because of dense macrophyte growth. A zigzag pattern of boat surveys combined with three to five longitudinal boat surveys spaced across the width of the river provided a good coverage of elevation points (95,654 points) of the navigable parts of the reaches. Walking surveys with a GPS rover added elevation points (1,069 points) in the shallow areas that were not navigable, at many top and toe topographic points along the river banks, and in areas in the Union Gap reach where dense macrophyte growth restricted the ability of the echo sounder to chart the channel bottom. Many erroneous echo-sounding points were deleted after careful review of each boat-survey trackline. The accuracy of the data is within the error bounds of similar studies, but the density of the data may be less than optimal for use in two-dimensional hydraulic models.