Scientific Investigations Report 2007–5007

Scientific Investigations Report 2007–5007

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Description of Study Area

The location and setting of the study area, the development of water resources in the basin, and an overview of the geology are presented to provide a general background for understanding the study area.

Location and Setting

The Yakima River Basin aquifer system underlies about 6,200  mi2 in south-central Washington (fig. 1). The Yakima River Basin produces a mean annual unregulated streamflow (adjusted for regulation and without diversions or returns) of about 5,600 ft3/s (about 4.1 million acre-ft) and a regulated streamflow of about 3,600 ft3/s (about 2.6 million acre-ft). The basin includes three Washington State Water Resource Inventory Areas (WRIA—numbers 37, 38, and 39), part of the Yakama Nation lands, and three ecoregions (Cascades, Eastern Cascades, and Columbia Basin—Omernik, 1987; Cuffney and others, 1997). The basin encompasses parts of four counties (Klickitat, Kittitas, Yakima, and Benton). Almost all of Yakima County, more than 80 percent of Kittitas County, and about 50 percent of the Benton County is in the basin. Less than 1 percent of the basin, principally in an unpopulated upland area, lies in Klickitat County.

The headwaters of the basin are on the upper, humid east slope of the Cascade Range, where the mean annual precipitation is more than 100 in. The basin terminates at the confluence of the Yakima and Columbia Rivers in a low-lying, arid area that receives about 6 in. of precipitation per year. Altitudes in the basin range from 400 to nearly 8,000 ft. Eight major rivers and numerous smaller streams are tributary to the Yakima River (fig. 1); the largest tributary is the Naches River. Most of the precipitation in the basin falls during the winter months as snow in the mountains. The mean annual precipitation over the entire basin is about 27 in. (about 12,300 ft3/s or 8.9 million acre-ft). The spatial pattern of mean annual precipitation resembles the pattern of the basin’s highly variable topography. The difference between the mean annual precipitation and mean annual unregulated streamflow is 6,400 ft3/s (about 4.6 million acre-ft); about 53 percent of the precipitation is lost to evapotranspiration under natural conditions.

The basin is separated into several broad valleys by large east-west trending anticlinal ridges. The valley floors are flat and slope gently towards the Yakima River. Few perennial tributary streams traverse these valleys. Most of the population and economic activity occurs in these valleys.

Agriculture is the principal economic activity in the Yakima River Basin. The average annual surface-water demand met by the Reclamation’s Yakima Project is about 2.5 million acre-ft; an additional 336,000 acre-ft of demand in the lower river basin is separate from the demand met by the project. Additional surface-water demand that is not met by Reclamation occurs in smaller tributaries and on the large rivers; this demand is based on State appropriated water. More than 95 percent of the demand is for irrigation of about 500,000 acres in the low-lying semiarid to arid parts of the basin (fig. 2). The demand is partly met by storage of nearly 1.1 million acre-ft of water in five Reclamation reservoirs. The major management point for Reclamation is the streamflow gaging station at the Yakima River near Parker. Just upstream of this site, at Union Gap, is the location that is considered the dividing line between the upper (mean annual precipitation of 7 to 125 in.) and lower (mean annual precipitation of 6 to 45 in.) parts of the Yakima River Basin. About 45 percent of the water diverted for irrigation is eventually returned to the river system as surface-water inflows and ground-water discharge, but at varying time-lags (Bureau of Reclamation, 1999). During the low-flow period, these return flows, on average, account for about 75 percent of the streamflow below the streamflow gaging station near Parker. Much of the surface-water demand in the basin below Parker is met by these return flows and not by release of water from the reservoirs. As a result of water use in the basin, the difference between mean annual unregulated and regulated streamflow in the basin is about 2,000 ft3/s, suggesting that some 1.4 million acre-ft of water, or about 17 percent of the precipitation in the basin, is consumptively used—principally by irrigated crops through evapotranspiration.

Development of Water Resources

Missionaries arrived in the basin in 1848 and established a mission in 1852 on Atanum (now Ahtanum) Creek. They were some of the first non-Indian settlers to use irrigation on a small scale. Miners and cattlemen immigrated to the basin in the 1850s and 1860s, which resulted in a new demand for water. With increased settlement in the mid-1860s, irrigation of the fertile valley bottoms began and the outlying areas were extensively used for stock rearing. One of the first known non-Indian irrigation ditches was constructed in 1867 and diverted water from the Naches River (Parker and Storey, 1913; Flaherty, 1975). Private companies later delivered water through canal systems built between 1880 and 1904 for the irrigation of large areas. The development of irrigated agriculture was made more attractive by the construction of the Northern Pacific Railway, which reached Yakima in December 1884 and provided a means to transport agricultural goods to markets; two years later, the completion of the railway to the Pacific coast provided new and easily accessible markets for agricultural products. The State of Washington was created in 1889, spurring further growth in the basin, especially because the cities of Ellensburg and Yakima were in contention for being the State capital. By 1902 there were about 120,000 acres under mostly surface-water irrigation in the basin (Parker and Storey, 1913; Bureau of Reclamation, 1999).

The Federal Reclamation Act was enacted in 1902 to enable the construction of Federal water projects in the western United States in order to expand the development of the West. In 1905, the Washington State Legislature passed the Reclamation Enabling Act, and the Yakima Federal Reclamation Project was authorized to construct facilities to irrigate about 500,000 acres. As part of the 1905 authorization and extensions, all forms of further appropriation of unappropriated water in the basin were withdrawn (Parker and Storey, 1913). Six dams were constructed as part of the Yakima Project: Bumping Dam in 1910, Kachess Dam in 1912, Clear Creek Dam in 1914, Keechelus Dam in 1917, Tieton Dam (Rimrock Lake) in 1925, and Cle Elum Dam in 1933. The construction of the dams and other irrigation facilities resulted in an extremely complicated surface-water system (fig. 3). These Federal reservoirs provide water storage to meet irrigation requirements of the major irrigation districts at the time of year when the natural streamflow from unregulated streams can no longer meet demands; this time is referred to as the ‘storage control’ date. Several of the reservoirs also provide instream flows during the winter for the incubation of salmon eggs in the salmon redds (gravel spawning nests).

Legal challenges to water rights resulted in the 1945 Consent Decree (U.S. District Court, 1945) that established the framework of how Reclamation operates the Yakima Project to meet water demands. The Decree determined two classes of rights—nonproratable and proratable. When the total water supply available (TWSA, defined as current available storage in the reservoirs, estimates of unregulated flow, and other sources that are principally return flows) is not sufficient to meet both classes of rights, the proratable (junior) rights are decreased according to the quantity of water available defined by the TWSA. This legally mandated method generally performs well in most years, but is dependent on the accuracy of the TWSA estimate. In some years, for example 1977, problems have arisen because of errors in the TWSA estimate (Kratz, 1978; Glantz, 1982). System management also accounts for defined instream flows at selected target points on the river, and for suggested changes in storage releases recommended by the Systems Operations Advisory Committee (SOAC)—the advisory board of fishery biologists representing the different stakeholders (Systems Operations Advisory Committee, 1999).

The drilling of numerous wells for irrigation was spurred by new (post-1945) well-drilling technologies, legal rulings, and the onset of a multi-year dry period in 1977 (Vaccaro, 1995). Population growth in the basin was, and still is, the driving force behind the increased drilling of shallow domestic wells and deeper public water supply wells. Currently, there are more than 20,000 wells in the basin. More than 70 percent of these wells are shallow, 10–250 ft deep, domestic wells. Based on the digital water-rights database provided by WaDOE (R. Dixon, Washington State Department of Ecology, written commun., 2001) and other information, there are at least 2,874 active ground-water rights associated with the wells in the basin that can collectively withdraw an annual quantity of about 529,231 acre-ft during dry years. The irrigation rights are for the irrigation of about 129,570 acres. There are about 16,600 ground-water claims in the basin; these claims are for some 270,000 acre-ft of ground water (J. Kirk, Washington State Department of Ecology, written commun., 1998). ‘A water right claim is a statement of claim to water use that began before the state Water Codes were adopted, and is not covered by a water right permit or certificate. A water right claim does not establish a water right, but only provides documentation of one if it legally exists. Ultimately, the validity of claimed water rights would be determined through general water right adjudications’ (Washington State Department of Ecology, 1998). A ground-water claim means a user claims that they were using ground water continuously, prior to 1945, when the State legislature enacted the Ground Water Code, for a particular use.

Overview of the Geology

The Columbia Plateau has been informally divided into three physiographic subprovinces (Meyers and Price, 1979). The western margin of the Columbia Plateau contains the Yakima Fold Belt subprovince and includes the Yakima River Basin. The Yakima Fold Belt is a highly folded and faulted region, and within the study area it is underlain by various consolidated rocks ranging in age from Precambrian to Tertiary, and unconsolidated materials and volcanic rocks of Quaternary age (fig. 4). In the Yakima River Basin, the headwater areas in the Cascade Range include metamorphic, sedimentary, and intrusive and extrusive igneous rocks. The central, eastern, and southwestern parts of the basin are composed of basalt lava flows of the Columbia River Basalt Group (CRBG) with some intercalated sediments that are discontinuous and weakly consolidated. The lowlands are underlain by unconsolidated and weakly consolidated valley fill comprising glacial, glacio-fluvial, lacustrine, and alluvium deposits that in places exceed 1,000 ft in thickness (Drost and others, 1990). Wind-blown deposits, called loess, occur locally along the lower valley.

Valley-fill deposits and basalt lava flows are important for ground-water occurrence in the study area. The basalt consists of a series of flows erupted during various stages of the Miocene Age, from 17 to 6 million years ago. Basalt erupted from fissures in the eastern part of the Columbia Plateau and individual flows range in thickness from a few feet to more than 100 ft. The total thickness in the central part of the plateau is estimated to be greater than 10,000 ft (Drost and others, 1990) with a maximum thickness of more than 8,000 ft in the study area. Unlike most of the Columbia Plateau, the CRBG in the Yakima Fold Belt is underlain by sedimentary rocks. The valley-fill deposits were eroded from the Cascade Range and from the east-west-trending anticlinal ridges that were formed by the buckling of the basalt sequence during mid- to late-Miocene time. Much of these deposits are part of the Ellensburg Formation. This formation underlies, intercalates, and overlies the basalts along the western edge, and comprises most of the thickness of the unconsolidated deposits (informally called the overburden; Drost and others, 1990) in the basinal areas. The basins are narrow to large open synclinal valleys between the numerous anticlinal ridges.

The deposition of a thick, upper sequence of sand, gravel, and some fine-grained material is the result of erosion by glacial ice and transport by meltwater streams. Damming of large lakes by glacial ice during the Pleistocene epoch resulted in the deposition of silt and clay beds in parts of the uplands. When the lakes drained, the fine sediments were exposed, subsequently eroded by wind, and deposited over the lower, eastern parts of the study area. Thus, the unconsolidated materials in the basinal areas that are abutting and interbedded with the basalts range in age from Miocene to Holocene.

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