Hydrology (River Flow)
Recent and projected changes in temperature and precipitation have, and will continue, to alter the hydrologic behavior of the Skagit basin. Basin-wide, the Skagit will shift to a more rain dominant behavior by the end of the 21st century, which is indicated by a change in the timing of river flow (see figure to the right). The overall mechanism of these shifts in river flow is primarily attributable to reductions in snowfall due to higher temperatures (the precipitation will fall as rain), as opposed to a change in the amount of precipitation. Warmer temperatures and a higher proportion of winter precipitation falling as rain, rather than stored as snow, will reduce the amount of water stored as peak snowpack (the April 1 snow water equivalent (SWE)) in the Skagit Basin overall) increasing river flow in winter and decreasing flow in summer (see figure to the right). The largest losses of snowpack are projected for mid-elevation basins, where average winter temperatures historically are near freezing. The timing of river flow at colder, upper Skagit sites near Ross Lake is less sensitive to projections of warmer temperatures, as precipitation will continue to be stored as snow. Areas in the lower Skagit Basin, like Mt Vernon, will undergo pronounced changes in the timing of river flow as temperatures rise in the 21st century, however annual flows will probably stay about the same or may even increase slightly on average. By the 2080s, peak monthly river flow is projected to shift from June to December in the lower basin, as a result of more precipitation is falling as rain and contributing directly to runoff production.
Not only is the average monthly river flow expected to change but projections for extreme events (i.e. flooding and low flows) become more intense in the future for the Skagit River. The increase in winter runoff, resulting from more precipitation falling as rain rather than being stored as snow, is projected to increase the frequency of the 100-year flood under natural conditions (~200,000 cfs at Mt Vernon for the current climate) by about 30%. These changes imply that the historical 100-year flood event would become more frequent. On the other end of the extreme flow spectrum, the loss of snowpack and drier summers will boost the severity of summertime low flows. The projected loss of melt water from glaciers in the late summer is expected to exacerbate low flow impacts.