Why are thousands of migratory salmon dying before they can spawn?

Portland, Ore. — Unseasonably hot water has killed nearly half of the sockeye salmon migrating up the Columbia River through Oregon and Washington state, a wildlife official said on Monday.

Only 272,000 out of the more than 507,000 sockeye salmon that have swum between two dams along a stretch of the lower Columbia River have survived the journey, said Oregon Department of Fish and Wildlife fisheries manager John North.

http://www.csmonitor.com/USA/2015/0727/Why-are-thousands-of-migratory-salmon-dying-before-they-can-spawn