Like many hydroelectric reservoirs, Ross Lake experiences considerable fluctuations in its water level throughout the year. The annual water cycle for Ross Lake is largely dependent on mountain snowpack. During winter months, most of the precipitation that falls on the upper Skagit watershed is normally in the form of snow. The water from this precipitation remains locked in the mountains until the spring melt, with little water flowing into the reservoir during these months.

Seattle City Light (SCL) works to meet several mandates for the Skagit Hydroelectric Project – maintaining sufficient river flow for fish spawning downstream of the project, supporting recreational opportunities on the reservoirs created by the project, and of course, providing electricity for the city of Seattle. Ross Lake is a large storage basin for SCL. From late summer through early spring, water flow into the reservoir is low and the lake level is drawn down as less water flows in than the amount flowing out through the generating turbines. The utility endeavors to manage the water level in the reservoir so that winter drawdown is sufficient to accommodate spring runoff without the need to spill to prevent flooding above the treaty full pool elevation of 1602.5 feet/488.4m above sea level (i.e. letting water through the dam’s spill gates without producing electricity). SCL measures the snowpack in the mountains throughout the winter to predict the volume of spring runoff and the appropriate level of drawdown required over a particular winter. The utility aims to maintain the reservoir at or near full pool during the busy camping/recreation season from late June through Labour Day.

Due to the shallow grade of the glacially carved valley, a large amount of lake bottom is exposed during the drawdown period, revealing the stumps left from clearing the formerly forested valley bottom. Six miles/ten kilometres or more of lake bed may be exposed by the time the lowest water level is reached in April.

As with most reservoirs, the trees were logged from the area slated for flooding in order to harvest the valuable timber, reduce the amount of debris that would accumulate at the dam, and remove obstacles/safety hazards for recreational activities. The Silver-Skagit Road which provides access to the north end of Ross Lake today was initially built as the logging road to remove the timber from the bottom of the reservoir. Logging crews were hired from the corresponding side of the border, and despite all wood exiting the valley via Canada, logs were sent to mills in their country of origin.

As climate change intensifies, the precipitation patterns of the North Cascades are becoming less predictable. Extreme fluctuations in snowpack and accumulation scenarios that don’t conform to historic patterns are making it more difficult for SCL to manage the water level in Ross Lake. The winter/spring of 2019 was particularly unusual, resulting in the lake level peaking 36 feet/11m below full pool in July. This left the lake bed exposed well south of the international border, prohibiting the use of all boat launches and significantly impacting recreational access and activities on the reservoir.

Photos: AJ Fedoruk