Smoke from wildfires in eastern Washington, together with some late August rainstorms, may have helped salmon return to the Nisqually River. Smoke drifting westward over the Cascades apparently blocked enough sun to break up the heat wave that was warming the river.
Last week, we wrote about a real-time temperature monitoring project by the Nisqually Tribe.
A group of state agencies, the Nisqually Tribe and Tacoma Power agreed to lower flows from the Alder reservoir into the lower Nisqually. Low snowpack, lack of rain, and little flow into the reservoir left a layer of warm water on top of a deeper pool of cooler water.
It was only a matter of time until that warm surface water would get dumped into the river if Tacoma Power followed previously agreed-upon minimum flows.
Even worse, the power utility would be in a situation with the river running free through the reservoir. Because the reservoir lacks trees to provide shade, water temperatures would have increased sharply.
Warm water is harmful to fish. Salmon stop migrating when water is warmer than 70 degrees. They also get stressed, becoming more likely to die before spawning and are more susceptible to disease.
The tribe, Tacoma Power and the state agencies agreed to go below the minimum flow requirements to make the cool water in the pool last the long, hot summer. At the same time, the tribe began to extensively monitor temperatures in the lower river.
What they found was that temperatures were bound to spike during heat waves. A series of long stretches of sunny and hot August days drove the temperature of the river up.
The tribe tracked the effect of one smoky weekend and a series of rainstorms in late August on river temperatures.
In this chart of water temperatures at various locations in the lower river, you can see significant dips correlating with major rainstorms or wildfire smoke.
So far, the balancing act by Tacoma Power and their watershed partners to meter out the cold water in the reservoir (while also maintaining enough water for fish to survive) has been working. Now, as the supply of cool water in the reservoir is getting lower, the river will be able to cool itself before it reaches migrating salmon.