VERIFICATION OF NEGLIGIBLE CONTRIBUTIONS OF MELT WATER FROM ROCK GLACIERS TO WATER SUPPLY IN FOUR CENTRAL ANDEAN WATERSHEDS, SAN JUAN PROVINCE, ARGENTINA

For the second consecutive year, we examined the headwaters of the El Altar, La Pantanosa, Rio Atutia and Rio Salinas watersheds, Calingasta County, San Juan Province, Argentina, to evaluate the melt water contributions to water supply from headwater rock glaciers. Proceeding downstream from the rock glaciers, vegas (springs), and streams were gauged, measured for field parameters, and sampled for geochemical analyses. Interstitial ice was sampled from two rock glaciers, which were slightly acidic low ionic strength solutions, with specific conductances of <10 mS/cm. All vega flows downhill of rock glaciers would be best described as trickles. Spring waters were typically strongly acidic, and all had elevated specific conductance of >100 mS/cm. Thus, the spring waters arising from the first vegas down valley from the rock glaciers show no decipherable influence from the melting of perennial ice in any basin. Moreover, they deliver very small flows, at most 1 L/s. Travel downstream many kilometers in each watershed demonstrated that discharge increased by several orders of magnitude, with flow arising from baseflow contributions of vegas well removed from rock glaciers. Direct observation with gauging of springs and trunk streams supports this assertion. All streams displayed increased conductivity, demonstrating mixing with groundwaters, whose generally acidic compositions reflect interaction with hydrothermally altered volcanic bedrock. Thus, observation of both flow and geochemistry demonstrate that no discharge from any rock glacier could be deciphered in the streams down valley. Given that the high central Andes remain in drought, with accompanying low stream flows and virtually no overland flow occurring from snowmelt, completion of these surveys occurred during an optimal time for detecting point source contributions to flow, such as meltwater from perennial ice from rock glaciers. Even under these favorable conditions, no discharges from rock glaciers could be verified. We are also awaiting analyses of hydrogen and oxygen isotopes in these waters. Thus, we state with confidence that the rock glaciers in these watersheds contribute undetectable and negligible contributions to the watershed yields measured. Indeed, groundwater flow from vegas contributes the most to alpine water supply.