Gordon Grant

US Forest Service - Pacific Northwest Research Station

Co-Authors: Sarah Lewis, Anne Jefferson, Christina Tague, Michael Farrell

Abstract: Spatial patterns of summer streamflow in the Oregon Cascades vary dramatically between the geologically distinct High and Western Cascade regions. A key control on streamflow response between these two regions is the partitioning of water input between fast-draining shallow subsurface flow networks (Western Cascades) versus a slow-draining deeper groundwater system (High Cascades). These differences result from extremely high contrasts in rock permeability, porosity, and drainage density, and produce distinctly different patterns in magnitude, timing and temperature of streamflow. We consider how these geologically-based differences in groundwater storage capacity can significantly alter streamflow response to projected climatic warming. Climate warming in the Cascades will likely result in a higher proportion of precipitation falling as rain, rather than snow, resulting in diminished snowpacks that melt earlier. Shifts to earlier winter recharge in the High Cascades will result in an earlier summer streamflow recession, and less cold, late summer sustaining streamflow. In the Western Cascades, where there is already virtually no groundwater storage, and very warm and low summer streamflows, permanent streams may become intermittent or ephemeral. Taken together, these results imply that current models linking climate and streamflow changes need to account for differences in groundwater storage as a first-order control. Differences in sensitivity to climate change between areas with and without large groundwater reserves have major implications for water resources management, but are not well-integrated into current plans. In particular, the sustained cold stream temperatures necessary for bull trout and other aquatic species are a direct consequence of the underlying geology. The geologic framework of the Oregon Cascades must be integrated into any future assessment of bull trout habitat.

Video Length: 20 Minutes, 39 Seconds

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Geological framework for interpreting streamflow and temperature regimes under climate warming