Appraising Surface Water Quantity and Quality in the Upper Arkansas River Basin in Chaffee County, Colorado

Hydrology Days 2011 Appraising Surface Water Quantity and Quality in the Upper Arkansas River Basin in Chaffee County, Colorado G. H. Steed1, T. K. Gates, J. D. Niemann and J. W. Labadie Department of Civil and Environmental Engineering, Colorado State University Abstract. The Arkansas River, one of four major rivers that begin in Colorado, has the largest drainage basin in the state and serves as a vital source of water. At peak flows, the Arkansas River swells by 70% through Chaffee County, in the heart of the Rockies, and only increases by 20% before exiting the mountains onto the high plains near Pueblo. The production of this discharge depends on a complex natural system of snowmelt and groundwater-stream interactions, as well as on numerous human interventions. The Arkansas valley in Chaffee County, from Granite to Salida, is characterized as an intermountain, high altitude, agricultural, small-municipal, and recreational region. Increasing demands on the stream-aquifer system have necessitated more up- to-date, integrated, and technology-driven data to assist water users, managers, and decision makers in moving toward optimal overall water use. Monthly synoptic measurements and triannual water sampling over July 2009 – January 2011 at 19 river and tributary sites provide data on discharge; dissolved oxygen; pH; specific conductance; oxidation reduction potential; temperature; and major dissolved ions, uranium, selenium, and other select minerals. Data analysis for the range of hydrologic conditions during the study period reveals flow conditions similar to long-term averages. Relationships of specific conductance and total dissolved solids derived from field samples yields equations useful for estimation of mass loading in the river system. Spatial and temporal analysis of tributary to river discharge and dissolved solids loading helps quantify and qualify water sources as well as potential aquifer interactions. Overall water quality is good, with some constituents worthy of continued active monitoring. 1. Introduction Water is a critical resource for the arid Western United States, including Colorado, and the challenge of ensuring the viability of that resource into the future is escalating. Demand is increasing for the combination of municipal, industrial, and agricultural uses; new water supplies are becoming more difficult and expensive to develop; climate change and highly variable snowpack conditions seem apparent; there is a growing group of stakeholders interested in in-stream flows for recreational and ecological purposes; and population is increasing. In 2004, the Colorado Water Conservation Board (CWCB) began releasing the results of an ongoing study regarding the state’s water resources supply and demands. That study, called the Statewide Water Supply Initiative (SWSI), projected population growth of 25% to 99% in Colorado river basins over the period from 2000 to 2030. Water demands for the same period are projected to surge between 20% and 84% (CWCB 2004). The SWSI 1 Department of Civil and Environmental Engineering Colorado State University Fort Collins, CO 80523-1372 Tel: (970) 491-5387 e-mail: [email protected] Steed et al. report called to action stakeholders and researchers to prepare to meet those projected needs. In the Arkansas River Basin, the largest drainage basin in the state (Figure 1), projections show a 55% population increase from 2000 to 2030, producing a 38% increase in gross water demand (CWCB 2004). In the portion of the basin above Pueblo Reservoir, known as the Upper Arkansas River Basin (UARB), the population is projected to increase by 72% within that same time period, resulting in a 60% increase in gross water demand (CWCB 2004). Figure 1. River Basins of Colorado showing the study area in central Colorado in the upper headwaters of the Arkansas River Basin. 1.1 Objectives Better management of the water resources in the Arkansas River Basin will be required to meet increased future demands, and improved management will require more data collection and analysis of this complex water system. To that end, this report aims to describe the basic water quantity and quality characteristics of the Arkansas River Basin in the critical headwaters area of Chaffee County based upon data gathered over a recent 18- month period. Specifically, this report aims to analyze hydrologic conditions of river and tributary flows, explore seasonal variations of tributary-supplied flow and mass load contributions to the system, and evaluate basic surface water quality, all with a view towards determining priority hydrologic characteristics for a future decision support system. The outline of the analyses in this study is as follows. First, the conditions during the study period will be compared to long-term hydrologic conditions to provide context 156 Appraising Surface Water Quantity and Quality in the Upper Arkansas River Basin for the results of the study. Second, a water quantity analysis will be made to evaluate the contribution of tributaries and other sources to the flow in the Arkansas River. Third, an examination will be made of dissolved mass load contributions from these sources. Finally, an analysis of water quality will be made to assess broad indicator characteristics such as temperature and dissolved oxygen, various tested metals, and other dissolved compounds. 1.2 Hydrologic Setting The study area (Figure 1) lies in Chaffee County and extends from the town of Granite in the north to Salida in the south. The valley floor of the Arkansas River is a relatively flat, deposit-filled inter-montane valley (Figure 2). It is the second northernmost structural basin of the Rio Grande Rift (Chapin and Cather, 1994). Tectonic rifting formed a deep structural basin bounded by normal faults, which has filled with alluvial, glacial, and other basin-fill deposits to a depth of about 5,000 ft (Watts 2005). On the west side of this valley is the Sawatch Range, which contains a large number of 14,000-foot mountain peaks and collects considerable snowpack during the winter. The Sawatch Range feeds four major, perennial tributaries to the Arkansas River within Chaffee County. On the east side of the valley is the Mosquito Range, which has 2 peaks over 13,000 feet in Chaffee County (most of the range crests near 10,000 ft) and receives much lower precipitation. The Mosquito Range feeds one small perennial stream north of Buena Vista, called Fourmile Creek. Precipitation is highly variable within the study region, ranging from about 10 in/yr on the valley floor to as much as 40 in/yr in the surrounding mountains, particularly the Sawatch Range (Crouch et al 1984). Runoff from precipitation, primarily snow in the Sawatch Range, is the sustaining source of tributary flow (Abbott 1985). Correlations between monthly average streamflow and total monthly precipitation are poor, but correlations between the April 1 snowpack water content and annual average streamflow are statistically significant (Burns 1985). Mean annual runoff is more than 30 inches at high elevations in the Sawatch Range, but it is 2 to 5 inches on the Arkansas Valley floor (U.S. Geological Survey 1970, as cited in Abbott 1985). The principal tributaries in the study region going from north to south consist of Clear Creek, Cottonwood Creek, Chalk Creek, and the South Arkansas River, all of which drain from the Sawatch Range (Figure 2). Smaller perennial tributaries include Pine Creek, Fourmile Creek, and Browns Creek. The tributaries flow across the alluvial, glacial, and basin-fill deposits of the valley floor to the Arkansas River, which is on the eastern edge of the valley. Tributaries are typically sources of groundwater recharge near the mountain front, loosing flow as they cross the valley floor. However, the tributaries represent groundwater discharge locations in their downstream reaches, closer to their confluence with the Arkansas River (Watts 2005). The Arkansas River and the South Arkansas River are gaining reaches due to discharge from the regional ground-water system (Watts 2005). Surface water diversions in Chaffee County are used primarily for agriculture including alfalfa and hay cultivation as well as irrigated pasture (Abbott 1985). Municipal public water supplies for the largest towns in the UARB (Salida and Buena Vista) also rely in part on surface water diversions (Watts 2005). 157 Steed et al. Figure 2. Study area and water monitoring network. 2. Methodology This section describes the data collection efforts that were undertaken to characterize the basic water quantity and quality characteristics of the study region. Data collection included in-situ measurements of discharge and other water characteristics, as well as laboratory analysis of water quality samples. Discharge data were collected to examine flow accumulation in the system and to calculate mass loading rates. In-situ measurements of basic water characteristics like temperature and dissolved oxygen were made to broadly 158 Appraising Surface Water Quantity and Quality in the Upper Arkansas River Basin evaluate water quality of different locations in the system. Finally, laboratory analyses of water samples were performed to measure the concentration of particular dissolved compounds and metals. 2.1 Selection of Sites for Data Collection Data collection sites were selected to observe both the main stem of the Arkansas River and its largest tributaries in Chaffee County (Figure 2). Routinely monitored tributaries include Clear Creek, Fourmile Creek, Cottonwood

Load more