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

We examined the headwaters of the El Altar, La Pantanosa, Rio Atutia and Rio Salinas watersheds, Calingasta County, San Juan Province, Argentina, to determine the potential melt water contributions to water supply from rock glaciers. Proceeding downstream from the rock glaciers, springs (vegas), tributaries, and the trunk streams were gauged, measured for field parameters, and sampled for geochemical analyses. Melt water from ice in rock glaciers should be low ionic strength, circum-neutral waters. At distances of several meters to hundreds of meters downstream from the landforms, the first presence of vegas, flowing or static, were observed. At this offset from the rock glaciers, spring waters were typically strongly acidic, and all had elevated specific conductance of 141-1300⁺ 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 revealed that discharge increased by several orders of magnitude, with additional flow arising exclusively from baseflow contributions of vegas topographically well removed from rock glaciers. Direct observation with gauging of springs, tributaries, and trunk streams supports this assertion. In addition, all trunk streams displayed increased specific conductance, demonstrating mixing with groundwaters, whose typical acidic compositions reflect interaction with the hydrothermally altered volcanic bedrock. Thus, observations 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 are currently in a dry hydroperiod, with accompanying low stream flows and virtually no observable overland flow from snowmelt, completion of these surveys occurred during optimal summer conditions for detecting point source contributions to flow, as would be expected from melting of perennial ice in rock glaciers. Even under these favorable circumstances, no discharge from rock glaciers could be verified. Thus, we state with confidence that the rock glaciers in these watersheds contribute undetectable and negligible contributions to their watershed yields.