Hydrogeology of the upper and middle Verde River watersheds, central Arizona

Blasch, Kyle; Hoffmann, John; Graser, Leslie; Bryson, Jeannie; Flint, Alan

The upper and middle Verde River watersheds in central Arizona are primarily in Yavapai County, which in 1999 was determined to be the fastest growing rural county in the United States; by 2050 the population is projected to more than double its current size (132,000 in 2000). This study combines climatic, surface-water, ground-water, water-chemistry, and geologic data to describe the hydrogeologic systems within the upper and middle Verde River watersheds and to provide a conceptual understanding of the ground-water flow system. The study area includes the Big Chino and Little Chino subbasins in the upper Verde River watershed and the Verde Valley subbasin in the middle Verde River watershed. The Big Chino subbasin, in the upper Verde River watershed is 1,850 square miles in area. Within the subbasin, Big Chino Valley and Williamson Valley encompass about 570 square miles excluding the surrounding mountains and the western part of the Coconino Plateau. The valleys are filled with alluvial deposits eroded from adjacent uplands and interbedded basalt flows. Median thickness of the combined alluvial deposits and basalt flows is about 435 feet. The estimated volume of saturated basin-fill deposits within the valleys is about 155 million acre-feet. Beneath the basin-fill aquifer is a sequence of water-bearing Paleozoic formations that receive recharge where they crop out along the western boundary of the subbasin. Together, the basin-fill sediments and the Paleozoic formations constitute the regional aquifer in the Big Chino subbasin. Water-balance calculations indicate that about 1–2 percent of annual precipitation recharges the regional aquifer. Recharge occurs primarily along the Juniper and Santa Maria Mountains, Big Black Mesa, Granite Mountain, and Bill Williams Mountain. Average winter base flow at the Williamson Valley streamflow-gaging station (09502800) was 3.9 cubic feet per second during 1965–84 and 1.7 cubic feet per second during 2002–03. The decline is attributed primarily to climate fluctuations. Base flow at the Verde River near Paulden streamflow-gaging station (09503700) averages about 17,700 acre-feet per year. It currently (2003) is about equal to the long-term average but has declined at an annual rate of about 380 acre-feet per year since about the mid-1990s. Ground-water outflow from the Big Chino Valley occurs only as base flow in the Verde River. Declines in ground-water altitudes during the past 50 years are attributed primarily to ground-water withdrawals. The Little Chino subbasin, in the upper Verde River watershed, is the smallest of the three subbasins in the study area and has had the greatest ground-water development. The regional aquifer underlying the subbasin is composed of sedimentary, volcanic, and basin-fill deposits of Quaternary and Tertiary age. Interfingering of less permeable units, such as lati-andesite, cemented alluvium, and trachyandesite, create confining conditions and artesian flow in some areas. The regional aquifer encompasses an area of about 310 mi2, including Little Chino Valley and Lonesome Valley. Thickness of the basin-fill deposits generally ranges from about 100 to 800 feet; the estimated volume of the saturated basin-fill deposits in the Little Chino subbasin is 33 million acre-feet. Winter precipitation is the primary source of recharge for the Little Chino subbasin as well as for other subbasins in the upper and middle Verde River watersheds. Water-balance calculations indicate that about 1–2 percent of annual precipitation recharges the regional aquifer in the Little Chino Subbasin. This amount may have been reduced since the predevelopment period (before 1940) by the construction of channel retention facilities along Granite Creek and its tributaries. During predevelopment times a larger volume of ground water (Abstract continued in Notes.)

Hydrogeology of the upper and middle Verde River watersheds, central Arizona

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