Salmon and steelhead of the Pacific Northwest are dependent on the delivery of abundant, cool water from seasonal snowmelt to support their migration, spawning, incubation, and rearing. The anthropogenic release of greenhouse gases, which is raising global air temperatures, poses a threat to the seasonal accumulation and melt of snow in the Pacific Northwest. As temperatures have warmed, the region has experienced a greater proportion of precipitation falling as rain and a lesser proportion falling as snow. It is projected that this trend will increase during the 21st century, likely causing reduced snowfall in most areas, more runoff during the winter, earlier peak streamflows during the spring, and diminished runoff during the summer, when water is most needed for salmon and other competing uses. The ceded areas of the member tribes of The Columbia River Inter-Tribal Fish Commission may be highly vulnerable to these changes.

We performed a Geographic Information Systems (GIS)-based analysis to better anticipate changes to snowpack on these tribal ceded areas. This analysis included the use of contemporary climate data and projections of 21st century climate change. Contemporary data were examined to determine the extent of areas near or just above the current mean winter freezing level, which may transition from snow-dominated to rain-dominated regimes with moderate warming. A snowpack model was constructed and implemented at monthly time steps to simulate precipitation, snowpack, and snowmelt over a distributed area during future climate scenarios.

The results of this analysis demonstrate that large portions of the tribal ceded areas are vulnerable to near-term climate change, especially in those subbasins that have a large amount of area at moderate elevations, and those that are further west and experience relatively mild temperature ranges. The results also identify higher elevation areas and areas in more eastern continental climates, which may be buffered from near-term temperature increases and could offer thermal habitat reserves for salmon as temperatures increase. An examination of modeled results showing monthly water balances from precipitation and snowmelt during future scenarios suggests how the timing of runoff may be affected in each subbasin, and generates data that can be used as an input to detailed hydrologic simulations.