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Wilson, John L ¹

¹ New Mexico Institute of Mining and Technology

The traditional hydrologic view of western mountains was constrained by the notion that the only significant natural water storage in mountains is snowpack, giving less attention to soil water storage, and essentially ignoring the role of groundwater. The only significant pathway for transmitting water through mountains was considered to be stream flow. Groundwater’s only role was as a contributor to stream base flow, which was empirically characterized through base flow separation calculations. Bedrock constitutes the bulk of a mountains mass, and this traditional view was conditioned by bedrock’s apparent imperviousness. That view has changed over the last fifteen years, and a much larger role for groundwater has emerged, both of processes within the mountain itself, and of mountains as a source of water to adjacent basins. A full view of the mountain system includes the entire mountain block, not just the thin veneer of soil and vegetation covering the bedrock. It includes the focused flow of mountain stream channels, but also the diffuse and focused movement of groundwater through fractured and faulted bedrock and, in sedimentary and certain volcanic rocks, through the rock matrix itself. This paper discusses this emerging view by reference to studies throughout the western states, with many of those studies aimed at the source of mountain groundwater, that is deep percolation into the bedrock from the shallow soil mantle, and a few aimed at how the recharge from deep percolation moves through the mountain block to eventually discharge at lower mountain streams or even the mountain front.