Types of Pacific Northwest Rivers
A basic overview of the hydrology of Pacific Northwest (PNW) rivers is very helpful in understanding some of the underlying physical mechanisms that link climate to streamflow. Most of the hydrologically significant precipitation we get in the PNW comes in the winter months from October-March. At other times of the year there is much less precipitation and what precipitation does fall is predominantly returned back to the atmosphere through evapotranspiration of moisture through plant root and leaf systems. So the winter months are crucial for determining what happens to PNW streamflow in any given water year. Different kinds of rivers, however, respond differently to this winter inflow of moisture to the region. Rivers at low elevation tend to respond quickly and directly to the precipitation that falls on the basin, since the basin temperatures are typically above freezing, and all the precipitation falls as rain. Rivers of this type are called rain-dominated and show a characteristic winter peak flow in their annual hydrograph (see Figure). Low-lying coastal rivers like the Chehalis River are rain dominated.
Many rivers on the slopes of the Cascades, like the Cedar River that supplies Seattle's water, are at moderate elevation and have a portion of the basin called the transient snow zone. The transient snow zone is an area in the basin where precipitation frequently falls as snow but then melts a few days or weeks later, a cycle that is typically repeated many times each winter. This transient snow zone can contribute to flooding if heavy rain and warm temperatures occur simultaneously when snow has accumulated (so called "rain on snow" events). Rivers of this type show both a peak in the winter and a peak in the spring and early summer (see Figure).
Snow melt dominated catchments are generally at higher elevations where temperatures are below freezing for most of the winter. In these basins, the winter precipitation falls predominantly as snow, where it is stored until the spring melt. Rivers of this type show a characteristic low flow period in the winter months, and a large peak flow in spring and early summer as the accumulated snow melts (see Figure). The Columbia and Snake Rivers are examples of snow melt dominated rivers.
Seasonal hydrologic response for three types of PNW rivers (normalized monthly average streamflow): snowmelt dominated, rain dominated, and mixed (transient snow).
Climate Change Impacts by River Type
Current projections of climate change for the Pacific Northwest (PNW) call for wetter winters and warmer temperatures throughout the year. The implications of these changes for the regional water cycle will vary according to river basin type:
- Snowmelt dominated basins would show reductions in witner snowpack, reduced summer and fall streamflows, and increased winter streamflow. The size of these changes would depend on the location of the specific river basin. Basins located significantly above today’s freezing level would be much less affected by warmer temperatures than those located at lower elevations. River basins whose present-day winter temperatures are close to freezing will be the most sensitive to climate change, due to the dramatic shifts in streamflow timing that would result from relatively small increases in wintertime temperatures.
- Transient (mixed rain and snow) basins would shift towards rain dominant behavior, with increases in flood risk in late winter, due to increases in both temperature and precipitation, and significant reductions in summer streamflows, primarily due to reductions in spring snowpack.
- Rain dominated basins would likely show increased winter streamflows resulting from increased precipitation, with less pronounced decreases in annual streamflow from temperature increases via increased evapotranspiration.”