Analyses indicate that water quality in the San Juan River drainage basin upstream from Shiprock, New Mexico, is quite variable from station to station. Analyses are based on water quality data from the U.S. Geological Survey WATSTORE files and the New Mexico Environmental Improvement Division 's files. In the northeastern part of the basin, most streams are calcium-bicarbonate waters. In the northwestern and southern part of the basin, the streams are calcium-sulfate and sodium-sulfate waters. Geology, climate, and land use and water use affect the water quality. Statistical analysis shows that streamflow, suspended-sediment, dissolved-iron, dissolved-orthophosphate-phosphorus, dissolved-sodium, dissolved-sulfate, and dissolved-manganese concentrations, specific conductance, and pH are highly variable among most stations. Dissolved-radium-226 concentration is the least variable among stations. A trend in one or more water quality constituents for the time period, October 1, 1973, through September 30, 1981, was detected at 15 out of 36 stations tested. The NASQAN stations Animas River at Farmington and San Juan River at Shiprock, New Mexico, record large volumes of flow that represent an integration of the flow from many upstream tributaries. The data collected do not represent what is occurring at specific points upstream in the basin, but do provide accurate information on how water quality is changing over time at the station location. A water quality, streamflow model would be necessary to predict accurately what is occurring simultaneously in the entire basin.
