The San Francisco Chronicle has some of the depressing details:
With state water resources constrained by the extreme drought, that’s the alarm that environmental, fishing and tribal groups are sounding after reports show the Sacramento River will reach dangerous temperatures during spawning season, based on federal scientific scenarios that analyze the bureau’s planned water releases. They warn of a massive die-off as bad as during the last drought, when 95% of winter-run chinook salmon eggs and young fish were wiped out in 2014 and 2015.
“I see in this a very clear need for urgent action to stave off disaster in pushing a species that is already trending toward extinction even closer,” said Rachel Zwillinger, water policy adviser for the nonprofit Defenders of Wildlife. “We haven’t seen the type of urgency that the situation demands from the agencies.”
The bureau is legally obligated to maintain temperatures below 56 degrees in the upper part of the Sacramento River, yet one scenario from the National Marine Fisheries Service, a division of the National Oceanic and Atmospheric Administration, projects it going over that for several months during the critical spawning and incubation period for both winter- and fall-run chinook. The scenario shows that high temperatures would wipe out 80% of the endangered winter-run salmon eggs, and ecologists say few survivors would make it to adulthood.