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Yakima Gap Bathymetry

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9722-00111 - Bathymetry and Near-River Topography of the Naches and Yakima Rivers at Union Gap and Selah Gap, Yakima County, Washington, August 2008

Problem - Yakima County is conducting a hydraulic and sediment-transport study of the lower Naches River and the Yakima River between Selah Gap and Union Gap. As part of this study, the Bureau of Reclamation (Reclamation) is constructing a sediment transport model that relies on a bathymetric survey conducted for Reclamation in 2004 by the USGS Washington Water Science Center (WAWSC). To enable completion of the sediment transport mode, Reclamation has requested that the 2004 survey be expanded to include three addition reaches. These are 1) about 1.5 miles on the Yakima River upstream of the confluence of the Yakima and Naches Rivers, 2) about 1000 feet up the Naches River from the confluence with the Yakima River, and 3) about 1 mile on the Yakima River above Wapato Dam

Objectives - The objective is to provide a complete coverage of bathymetry and near-river topography for Reclamation to complete its sediment transport model. The goal is to provide accurate elevation data with adequate density of data points that include some points on the banks and near-shore features that may not be well defined with the existing LiDAR-based topographic data.

Relevance and Benefits - An important part of the USGS mission is to provide scientific information to manage the water resources of the Nation, and to enhance and protect our quality of life. This study is consistent with the national USGS mission and goals and water-resources issues identified in the USGS Washington District Science Plan 2004. The study is being conducted to assist a sister Department of the Interior bureau with their study of a water-resource management issue.

Approach - A survey grade GPS was used in conjunction with a boat-mounted single beam echo sounder to survey the boat-accessible portions of reaches. Where possible, a series of diagonal and parallel transects of boat surveys were made to create a dense mass of survey points of the main stem of the river and any major side channels. Areas of river channels not accessible by boat or overgrown with macrophytes were surveyed by wading and walking in parallel lines to the river with GPS rovers. GPS base stations were set on nearby first- or second-order benchmarks to provide real-time signal corrections to the GPS rovers.

The horizontal position of the boat surveys will be derived from RTK GPS relative to survey control stations set along a baseline between fixed survey monuments with A- or B-order of horizontal accuracy. The manufacturer's reported horizontal accuracy for RTK GPS is plus or minus 0.033 ft plus 1 part per million (ppm) Root Mean Square (RMS) (Trimble, 2006). Horizontal coordinates are in the Washington State Plane South coordinate system, datum NAD83. The vertical position was determined from RTK GPS-derived altitude minus the antenna to echo sounder length minus the depth recorded by the echo sounder. The survey control station was first or second-order for vertical accuracy. The manufacturer's reported accuracy for RTK GPS is plus or minus 0.066 ft plus 1 ppm RMS.

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