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(3) U.S. Department of Agriculture, Science and Education '~ Administration (USDA, SEA)--historical (1976 through 1980) data on streamflows at selected study sites later collected by Twentymile Coal Company. (4) Colorado Department of Health (CDH)--water-quality data on the Yampa River upstream from Trout Creek. (5) Colorado Mined Land Reclamation Division (CMLRD)--Public files on Pittsburgh and Midway Coal Mining Company's (P&M's) data on streamflows and water quality on upper Trout Creek. Streamflow and water-quality monitoring sites used in this study are indicated in Figure 1. For calibration purposes in this study, an 8-year period of record from the 1976 through 1983 water years was designated for assess- ing the historical variability of ambient conditions of streamflow '~ and water quality. This can be compared to the 6-year period of record (1976 through 1981 water years) included in the previous Parker and Norris (1983) model study. For this 8-year historial period (1976 through 1983 water years), gaps occurred in many of the available streamflow records; also, streamflows for the entire study period had to be estimated at selected model nodes. For prediction purposes in this study, parts of the historical period or average conditions available to the current study then could be selected, as desired, for evaluating mine-related impacts, based upon data and information provided by TCC on their proposed mine plan. Water-quality data for designated nodes frequently were sparce and at best were collected intermittently over parts of the historical period of record. For this study, through use of regression functions related to streamflow or indicator water-quality variables, a detailed and comprehensive water-quality characteri- I~ zation was developed for purposes of the modeling analyses. Streamflow data fill-in procedures and use o ~ regression functions -3-