Low Streamflows on the Skagit River in Early Summer of 2019

The warm and dry spring of 2019 has raised concerns about water supplies in parts of WA state. It is unclear how much it is a coincidence, but the same topic was addressed in this space a year ago. This time we focus on a single stream, the Skagit River. The Skagit is the largest source of freshwater for Puget Sound, and is a managed river system subject to the competing demands of agriculture, hydropower, recreation, and its ecosystems. Regarding the latter, the Skagit River features runs of all five native species of Pacific salmon. These runs constitute a substantial fraction of those for Puget Sound as a whole, including an estimated 60% of the wild chinook salmon. They depend on suitable freshwater habitats, including sufficient flows during the drier summer months.

Recent conditions on the Skagit have been subpar, as illustrated by the time series of streamflow discharge at Concrete from 1 May through 30 June 2019 shown in Figure 1. The stage was set by a below-normal winter snowpack in the upper part of the Skagit River watershed. Specifically, the snow water equivalent (SWE) in the Northern Puget Sound basin on 1 April 2019 was only 73% of normal. There was not just less snow, but it also melted faster than usual. There was some particularly warm weather during the second week of May – Diablo Dam had 5 days with maximum temperatures at 80°F or above from 8-12 May with a peak of 88°F – resulting in the first peak in streamflow shown in Figure 1. There was a second peak about a week later associated with a brief period of decent rains during a relatively dry month for western WA. The Skagit streamflows went downhill (pun intended) in June to values mostly less than 50% of normal for the time of year. June had another pair of temporary upticks with melting snow at higher elevations presumably responsible for the first one (Diablo Dam recorded 94 and 95°F for maximum temperatures on 12 and 13 June, respectively) and a short period of substantial rains in northwest WA near the end of June for the second one.

We are inclined to also take the long view and towards that end consider the time series of mean streamflow for the month of June in the Skagit River at Concrete for the years 1925-2019, as plotted in Figure 2. The period from 1925 to the mid-1970s included plenty of interannual fluctuations with a lack of any systematic trend. The period from the late 1970’s to present has featured generally lower flows than those that prevailed in the first half of the record. The year of 2019 is definitely on the low side but not to the extent of 2015 and 2005; in quantitative terms it is the 5th lowest June streamflow on record. The overall tendency illustrated in Figure 2 is concerning because of the importance of sufficient summer water supply from the Skagit for a variety of purposes. The Skagit River system has major dams, and the streamflow at Concrete reflects in large part the releases upstream. We are unsure how much the long-term trend in Skagit streamflow represents changes in management practices versus the climate. The climate must be playing some role given the declines in winter snowpack and increases in temperature. But we can still enjoy years with abundant winter snow and plentiful summer water supplies, and we hope that we will be covering a different sort of topic here a year from now.