ABSTRACTGreen Lakes Valley is a high (3500 m elevation), mid-continental drainage at relatively low latitude (40°N) which appears to have been influenced by a variety of environmental changes in the past half-century. Important among these have been increased rates of deposition of strong acid anions from the atmosphere and a corresponding tendency toward acidification of surface waters. In response, a declining trend in acid neutralizing capacity (ANC) and in the pH of streamwater in the basin has occurred in the past 25 years. This trend is most marked at the highest (most alpine) elevations in the basin where there is little soil and vegetation cover but remains detectable, if less marked, at all elevations down to treeline (3250 m). It suggests that acidification (defined as a loss of ANC) of surface water will be widespread in the basin within a decade.