Spring 1999 Bulletin

Downstream Notes: Water Quality on the Lower Río Grande

Despite the size and significance of the Río Grande watershed, until recently few studies had examined water quality of the entire basin in detail. Since 1995, the US Geological Survey j(USGS) has been monitoring water quality in the Río Grande Basin as part of a National Stream Quality Accounting Network (NASQAN), designed to characterize sediment transport and chemicals in the nation's largest rivers and to assess the influence of human activity on water quality. In general, according to a recent USGS Fact Sheet (FS-083-98), water quality is good in the upper parts of the basin, but declines as the water moves downstream, principally because of agricultural return flow, a lack of effective waste-water treatment, and extensive year-round agriculture in the lower basin. Salinity is a major water-quality problem throughout the Basin, as is the presence of agricultural pesticides and heavy metals from extensive mining in New Mexico, and the proliferation of maquiladoras. Mercury, lead, selenium, and arsenic––all of which are toxic to animals and humans––have been found in riverbed sediments of the Río Grande/Río Bravo, Pecos River, and Arroyo Colorado, according to a 1997 USGS Fact Sheet. Other trace elements found include copper, manganese, barium and cadmium, several of which show increasing trends downstream. Among the organic compounds detected are PCBs, chlordane, methylene chloride, and chloroform. The Río Grande Basin is also under study as part of the Biomonitoring of Environmental Status and Trends (BEST) program, in which fish tissue and fluid are collected to assess physiological changes that indicate exposure to a chemical. Because fish are prey for many creatures, including raptors, mammals, and humans. assessing the effects of contaminants on fish can provide an indication of potential effects on other species. It is partly because fish are an indicator of the health of a river system that there is so much concern this spring for the survival of the endangered Río Grande Silvery Minnow in the Middle Río Grande. The minnow, which is on the endangered species list, is at risk if the middle stretches of river run dry this summer because of over diversion. Of the 16 fish once native to the Río Grande in New Mexico, only nine remain. Four minnow species similar to the silvery minnow are already gone. Bucket brigades are standing by to scoop up minnows from the river and bring them to safety upstream, and several environmental groups have filed suit to have reservoir waters released in order to maintain the habitat necessary for the survival of both the silvery minnow and the endangered willow flycatcher. Nevertheless, many expect the river will run dry. Data from the Río Grande NASQAN are available on the web at: http://water.usgs.gov/public/nasqan

Please return to Spring 1999 Bulletin Index.