It's my understanding that it is a mostly a water supply problem, occurring not only in central and E. WA, but also a number of watersheds in W. WA. In particular, low streamflows, thus warm water and dewatered habitat, are occurring because too much groundwater pumping is going on. This is both from lots of new development, and also some users which are pumping out huge amounts for various reasons. Maybe some of that is wasted, though even "wasted" water can sometimes make it's way back into the river.
Secondarily, clear cutting logging creates lots of young to middle aged forests that are growing really rapidly. That's great if you are a logging company, but just like teenagers, those young fast growing trees drink tons of water, more than old trees. That drops the water level in the river lots more than you might expect, trees basically pump water into the air.
Both those factors combined are thought to be contributing to too little water in the rivers and summer low/warm flow issues, and that's why water issues are really tightening up.
