The Highland Basins include the Salt River, Tonto Creek and Verde River basins, and the northern half of the Agua Fria Basin. Basin-fill aquifers in the highlands are limited in areal extent and are hydrologically connected with stream alluvium. Consolidated rock aquifers surround and underlie the basin-fill aquifers and contribute underflow. Basin-fill aquifers also receive inflow from stream infiltration and mountain front recharge. Where the basin-fill aquifers are discontinuous, underflow between them may be restricted (Anderson, et al., 1992).
Verde River Basin
The Verde River Basin is a relatively large basin that encompasses part of the Coconino Plateau in its northern portion with the Mogollon Rim defining its eastern boundary. It is characterized by steep canyons, rugged mountains and by broad alluvial valleys in the north and west-central portions of the basin. The basin is divided into the Big Chino, Verde Valley and Verde Canyon sub-basins as shown in Figure 5.5-6 and Figure 5.5-8.
Natural recharge and groundwater in storage estimates for the basin, sub-basins and local areas are listed in Table 5.5-6. Groundwater recharge estimates for the entire basin range from 107,000 AFA to more than 138,000 AFA. Groundwater in storage is estimated to range from 13 maf to more than 22 maf for the entire basin. Few water level measurements were taken in the basin in both 1990-‘91 and 2003-‘04 (Figure 5.5-6). Water level change measurements taken during different time periods are shown for the Big Chino Sub-basin (Figure 5.5-6A) and the Verde Valley Sub-basin (Figure 5.5-6B) and are discussed in the sub-basin sections below. Well yield varies throughout the basin with the most productive wells located in the Big Chino Sub-basin (Figure 5.5-8). The median well yield for the entire basin is 260 gpm reported on registration forms for 262 large (>10-inch) diameter wells.
A number of hydrogeologic studies of the Big Chino and Verde Valley sub-basins, and to a lesser extent the Verde Canyon Sub-basin, have been conducted and are briefly referenced here. These studies, many of them recent, contain detailed information about the groundwater and surface water systems in the basin and are referenced in this section and in the Verde River Basin references and supplemental reading. Each sub-basin is discussed below from north to south across the basin.
Big Chino Sub-basin
The Big Chino Sub-basin has an area of about 1,850 square miles. The principal aquifer consists of basin-fill sediments interbedded with volcanic rocks of Cenozoic age that fill the sub-basin. This basin-fill aquifer is commonly referred to as the Chino Valley Unit and is the major source of water for irrigation and domestic purposes. Chino Valley runs northwest to southeast from Seligman to Paulden. Well yields in Chino Valley wells are commonly greater than 1,000 gpm to greater than 2,000 gpm. A carbonate aquifer comprised of Paleozoic rocks underlies most of the Big Chino Valley Sub-basin and the area north of the Verde River near Paulden. It is assumed that there is a hydraulic connection between the two aquifers in the Big Chino Valley and the Williamson Valley, which runs north-south along the southeastern sub-basin boundary. The general location of aquifers and other features are shown in the graphic from Wirt, 2005.
Groundwater occurs under unconfined and confined (artesian) conditions in the basin-fill aquifer. Artesian conditions occur primarily where buried lava flows and coarse-grained sediments are interbedded with clays and volcanic ash. In the northwesternmost part of the sub-basin, basin-fill deposits may be as much as 2,500 feet thick. Further south and west of Paulden in the Williamson Valley, the thickness of the alluvium is estimated at 2,000 feet. In the eastern part of the Big Chino Sub-basin, the carbonate aquifer is the primary regional aquifer. This aquifer is dry west of the Mesa Butte Fault, which occurs north of Drake and runs northeastward, and between Williams and the Big Chino Valley (USGS, 2006). Alluvial sands and gravels along the major washes also yield water to wells and are utilized as a local water supply in the sub-basin.
Groundwater flow in the basin-fill aquifer is toward the Big Chino Wash drainage and then south. Groundwater flow in the carbonate aquifer is toward the north (Figure 5.5-6). Recharge occurs from mountain front recharge along the Juniper and Santa Maria Mountains on the west side of the sub-basin, from Granite Mountain on the south and from Big Black Mesa and Bill Williams Mountain on the east side and from runoff in major washes. Recharge also occurs via groundwater inflow from the Little Chino Sub-basin (Prescott AMA) north of Del Rio Springs. In 1999, this groundwater inflow was estimated at 1,800 AFA (Nelson, 2002). The Williamson Valley and Paulden areas are the most arid regions in the Verde River Basin.
Groundwater outflow from the Big Chino Sub-basin occurs as base flow in the Verde River and is currently estimated at about 17,700 AFA. Base flow at the Verde River near Paulden (gage number 9503700, see Figure 5.5-4) has declined at an annual rate of about 380 AFA since the mid-1990s (USGS, 2006). The average annual recharge volume for the sub-basin was estimated at 30,300 AFA for the period 1990-2003 (Blasch and others, 2006). McGavock (2003) estimated that there was 10 maf of groundwater in storage in the sub-basin to a depth of 1,200 feet bls.
Figure 5.5-6A shows water level changes in the sub-basin from 1992 to 2003-‘04 and water level elevation during 2003-‘04. More than half the wells measured showed some decline although water level increases of more than 15 feet were measured in wells south of Seligman. Well yields exceeding 2,000 gpm are found along the Big Chino Wash drainage (Figure 5.5-8). Water quality is generally good in the sub-basin with some occurrence of arsenic at levels that equal or exceed the drinking water standard in wells in the Paulden area.
Verde Valley Sub-basin
The Verde Valley Sub-basin is the largest sub-basin in the Verde River Basin with an area of about 2,500 square miles. The principal aquifer is the Verde Formation, which consists of a thick sequence of tertiary limestones and sandstones. The estimated depth of the formation reaches 4,200 feet based on aeromagnetic and gravity data (USGS, 2006). The formation flanks the Verde River for some distance from the Camp Verde area to north of Cottonwood. Other aquifers include the carbonate aquifer and an alluvial aquifer located along the Verde River. The carbonate aquifer, primarily sandstone of the Supai Formation and the underlying Redwall and Martin limestones is the main groundwater supply for Sedona. Locally perched groundwater in fractured or decomposed granite and in volcanic rocks provide small amounts of water in many locations. Groundwater occurs primarily under unconfined conditions although confined conditions occur locally within the Verde Formation. All three aquifers are hydraulically connected.
Click to view Table 5.5-6 Groundwater Data for the Verde River Basin
Click to view Figure 5.5-6 Verde River Basin
Click to view Figure 5.5-6A Big Chino Sub-basin Groundwater Conditions
Click to view Figure 5.5-11B Verde Valley Sub-basin Groundwater Conditions
Click to view Figure 5.5-7 Verde River Basin Hydrographs Showing Depth to Water in Selected Wells
Click to view Figure 5.5-8 Verde River Basin