A ninety-degree afternoon can tempt anyone to jump into a Sierra reservoir, but the water waiting below the surface is often cold enough to kill. Many California lakes—especially the big, snow-fed reservoirs and alpine basins such as Lake Tahoe—hold water that rarely climbs above 70 °F, even in July and August.
That deceptively low temperature triggers cold-water shock, a physiological response experts say is responsible for a large share of the state’s open-water drownings.
What cold-water shock does to the body
When someone plunges into water below roughly 70……