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Yakima Gap Bathymetry

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The Yakima County Flood Control Zone District for Yakima County, Washington is currently (2009) conducting a levee setback and floodplain restoration study on the Yakima and Naches Rivers in the area between the two prominant landscape features of Selah Gap and Union Gap that lies in and near the Cities of Yakima, Selah, and Union Gap, Washington. As part of this study, Yakima County requested the U.S. Bureau of Reclamation (Reclamation) to complete a sediment-transport model that was partially developed for a water-storage study recently completed by Reclamation. The sediment-transport model requires river bathymetry and topographic data on the floodplain.

For most of the reach, the data had already been acquired from several sources. The U.S. Geological Survey (USGS) Washington Water Science Center (WAWSC) conducted a river survey in portions of the reach in the summer of 2004 and in 2005, the USGS Columbia River Research Laboratory conducted river surveys in the area as part of Reclamation’s Storage Study (Bureau of Reclamation, 2008). Airborne LiDAR (Light Detection and Ranging) elevation data for the above-water portions were collected in 2000 by Horizons, Inc. and Yakima County conducted surveys on the Naches River sometime prior to 2005. However, the bathymetry for a portion of the reach near Selah Gap and near Union Gap was never surveyed and these data were needed to complete the sediment-transport model. The WAWSC surveyed the missing portions of the Yakima and Naches Rivers in August 2008 to provide a complete topographic and bathymetric data set for the sediment-transport model.

Water in the river reaches was relatively shallow, which limited boat access to small jet boats such as the one used for this survey. The Naches River reach had the steepest slope with the coarsest bed material composed of cobbles and small boulders. Boat access was limited to the lower portion of the reach, but the reach was wadable in most locations where the boat could not be used and a handheld GPS rover unit was used to conduct the survey. The Yakima River in the Selah Gap reach has about half the slope of the nearby Naches River reach with a bed material generally ranging from sands to gravels. The Yakima River in the Union Gap reach was the flattest reach and it was terminated with the Wapato and Sunnyside Diversion dams. There was fine-grained sand and mud along much of the edges of this reach and several areas support extensive macrophyte colonies that limited the use of the boat-mounted echo sounder. These areas were surveyed with a handheld GPS rover. Thick, thorny brush and cottonwood trees lined much of the banks of all the reaches which impeded walking access to a large portion of the channel margins.

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