Introduction

The Flood Control Zone District for Yakima County, Washington, is studying levee setback and floodplain restoration for the Yakima and Naches Rivers between Union Gap and Selah Gap, which are two prominent mountain-range gaps in or near the cities of Yakima, Selah, and Union Gap, Washington (fig. 1). As part of this study, Yakima County requested that the Bureau of Reclamation (Reclamation) complete a sediment-transport model that had been partially developed by Reclamation for a recently completed water-storage study. The sediment-transport model requires river bathymetry and topographic data for the floodplain.

Data for most of the reaches in the Yakima County study had been acquired from several sources. The U.S. Geological Survey (USGS) Washington Water Science Center (WAWSC) surveyed the Yakima County study area of the reach in summer 2004, and in 2005 the USGS Columbia River Research Laboratory surveyed the area as part of Reclamation’s water storage study (Bureau of Reclamation, 2008). Airborne Light Detection and Ranging (LIDAR) elevation data for the areas above water were collected in 2000 for the entire Yakima County Study reach by Horizons, Inc., and Yakima County surveyed the Naches River prior to 2005. The bathymetry for the Yakima River at Union Gap and the Yakima and Naches Rivers at Selah Gap, however, never was surveyed and these data were needed to complete the sediment-transport model. The multiple reaches of the Yakima and Naches Rivers that lacked data were surveyed by the USGS in August 2008 to provide a complete topographic and bathymetric data set for the sediment-transport model. The data-collection techniques were similar to those used in a study of the Kootenai River, Idaho (Barton and others, 2004).

Purpose and Scope

This report presents the bathymetric and topographic survey data that were collected and describes methods used to collect the data, and the accuracy of the data collected. The project area is three river reaches, one reach on the Naches River and two reaches on the Yakima River, where bathymetric and topographic data are lacking because the area was not surveyed in previous studies. The study area was in and near Union Gap and Selah Gap, two mountain-range gaps in Yakima County, Washington.

Study Area

The river reaches at Union Gap and Selah Gap included in this study (fig. 1) were surveyed in August when streamflow was average. The average annual streamflow (water years 1990–2007) is 36.0 m³/s (1,270 ft³/s) and the average August streamflow is 50.7 m³/s (1,790 ft³/s) upstream of the study area at the Reclamation-operated stream-gaging station, Yakima River below Roza Dam. The average annual streamflow is 46.4 m³/s (1,640 ft³/s) and the average August streamflow is 15.5 m³/s (548 ft³/s) at the Reclamation-operated stream-gaging station, Naches River near Yakima (fig. 1). During the survey August 12–14, 2008, the daily flow at the Yakima River below Roza Dam stream-gaging station varied from 56.5 to 59.8 m³/s (1,994–2,112 ft³/s) and at the Naches River near Yakima stream-gaging station the daily flow varied from 10.5 to 12.1 m³/s (371–428 ft³/s) (Yakima Hydromet Archive Data Access web site, accessed January 2009, http://www.usbr.gov/pn/hydromet/yakima/yakwevarcread.html).

Water was relatively shallow in the river reaches, which limited boat access during the survey. The lengths and slopes of the three reaches surveyed are shown in table 1. The steepest slope with the coarsest bed material (cobbles and small boulders) was in the Naches River reach. Boat access was limited to the lower area of the reach. In most locations where the boat could not be used, the reach was wadable and a handheld GPS rover unit was used to survey the river. The Yakima River in the Selah Gap reach has about one-half the slope of the nearby Naches River reach and bed material generally ranges from sands to gravels. The Yakima River in the Union Gap reach was the flattest reach and terminated with the Wapato and Sunnyside Diversion dams. Fine-grained sand and mud extend 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 part of the channel margins.