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A model for determining cumulative effects of streamflow, dissolvedsolids concentration, and dissolved-solids load was calibrated for the Yampa River and its tributaries in northwestern Colorado. The model uses accounting principles. It establishes nodes on the stream system and sums water quantity and quality from node to node in the downstream direction. The model operates on a monthly time step for water years 1976 through 1981. Output is monthly mean streamflow, dissolved-solids concentration, and dissolved-solids load. Streamflow and dissolved-solids data from streamflow-gaging stations and other data-collection sites were used to define input data sets to initiate and to calibrate the model. The model was calibrated at four nodes and produced results that generally were within 10 percent of the measured values. The calibrated model can be used to compute changes in streamflow and dissolved-solids concentration, and load resulting from the cumulative effects of new coal mines or the expansion of old coal mines in the Yampa River basin. INTRODUCTION Coal mining has increased in northwestern Colorado in response to the increased need for energy in the Nation. Because of the accessibility of coal and the availability of transportation, many coal mines have been established in the Yampa River basin. In addition, some mines are being expanded, and new sites have been proposed for mining. As mining activity increases in any river basin, there is concern regarding the cumulative effects of mines on the water resources. A concentration of mining activity ultimately can produce combined effects larger than any predicted effects from a single mine. Computing these cumulative effects is difficult because the combined effects of several mines may be very complex. The difficulty in determining the effects of mining is evident in the variability of the dissolved-solid concentrations and loads in streamflow draining these areas. Changes can cause the water to become unsuitable for many uses. In addition, a major concern is that any increase in dissolved solids in the Yampa River main stem may add to the problem of increasing dissolved-solids concentrations and loads in the Colorado River. A model to compute the dissolved-solids concentrations and loads in the Yampa River basin provides a method to examine the combined effects on water quality from several mines in the basin. The purpose of this report is to describe a calibrated model for simulating streamflow and dissolved solids in the Yampa River and its tributaries. The model was calibrated in cooperation with the U.S. Bureau of Land Management as one method of combining proposed coal mining with water-resource information for evaluating proposed leasing of tracts for coal mining. This report will: (1) Describe the study area; (2) describe the model; (3) identify the available data; (4) describe the data used in the model; (5) describe model calibration results; and (6) identify data needs. This model uses available data from the basin and is calibrated to existing conditions. Output from the calibrated model presented in this paper is assumed to represent 1976-81 water-year conditions. Alterations to the model easily can be made to represent particular hypothesized mine development during this same time period. The comparisons between these 1976-81 conditions and a particular mining situation provide the assessment of the cumulative effects during the 6 years. These 6 years are assumed to be representative of a typical sequence of years in terms of the hydrology in this basin. This period contains both wet and dry years and enables the user to examine a wide range of hydrologic conditions. An example of how this model is used in assessing cumulative hydrologic effects is given in Parker and Norris (1983). THE STUDY AREA The Yampa River basin (fig. 1) is hydrologically typical of many coal regions in Colorado. Much of the streamflow of the main stem of the Yampa River is derived from snow in the mountains. The geology of these mountains is different from the geology in the basin where the coal areas are located. The basic rock types in the mountains are igneous and metamorphic and result in streamflow that has small dissolved-solids concentrations. Coal mines are located in sedimentary-rock areas that may yield water with large dissolved-solids concentrations. The streams within the active coal area can be classified into two groups minor tributaries and major tributaries. Flow in the minor tributaries draining sedimentary-rock basins may contain naturally occurring large concentrations of dissolved solids. In general, most mining activity occurs in these small basins. The major tributaries receive water from several minor tributaries and carry the water to the Yampa River. Water use increases substantially in these major tributaries. The study area includes a drainage area of 2,806 square miles between the streamflow-gaging stations--Yampa River at Steamboat Springs (09239500) and Yampa River near Maybell (09251000)--in Moffat and Routt Counties of northwestern Colorado (fig. 1). About 68 percent of the annual mean streamflow at the streamflow-gaging station Yampa River near Maybell (09251000) originates upstream from the confluence of the Elk and Yampa Rivers just downstream from Steamboat Springs. Downstream from this confluence, precipitation and topographic relief decrease considerably. The Williams Fork, Elkhead Creek, Milk Creek, Fortification Creek, and Trout Creek are the major tributaries that contribute most of the remaining flow. The majority of this additional water comes from Williams Fork and Elkhead Creek. Most of the coal-mining activity is south of the Yampa River. To the north of the river, coalbeds dip farther underground, which increases the overburden and diminishes the potential for coal-mine operations. B as e fr o m U .S . G e o lo g ic a l S u rv e y, S ta te B as e M a p , 1 50 0. 00 0, 19 80 I I 5 1 0 15 K IL O M E T E R S F ig u re 1 .-S tu d y a re a . Collection of streamflow and associated water-quality data primarily was limited to the Yampa River prior to water year 1976. Data collection on the Yampa River was important because it is the primary surface-water resource of northwestern Colorado and is an important link in the Colorado River system. As interest grew in understanding water resources in the coal areas, additional streamflow-gaging stations and miscellaneous sites were established. The U.S. Bureau of Land Management funded a large part of this data-collection activity from 1976 through 1981 (Maura, 1982). This period (water years 1976 through 1981) has the most complete record of hydrologic data for the coalmining areas; therefore, the data for the model in this report are based on this 6-year period. Streamflow for the period of record for Yampa River near Maybell (09251000) is shown in figure 2. As shown, streamflow during the 6-year study fluctuated considerably and is representative of the variability within the period of record. The annual mean streamflow of 477 cubic feet per second during 1977 was the smallest of record. Annual mean streamflow for the study

Calibration of a dissolved-solids model for the Yampa River basin between Steamboat Springs and Maybell, northwestern Colorado