There was the whitefish die off in the Yellowstone a few years ago.
Generally there is about 50% mortality in sockeye salmon from Wells Dam to Zosel Dam on the Okanogan River. Some of this is from the Brewster pool fishery at Okanogan/Columbia confluence but there must be a pretty significant drop out when traveling through the river after the thermal barrier breaks as well. I'm sure some of that is related to stress, low oxygen/high temps, and disease.
I was there on the SF Nooksack early September to get samples for an eDNA project that north sound TU is doing. I was shocked to see all those dead unspawned salmon. I can only imagine how many thousands of unfertilized eggs were in those carcasses. Such a pity.