The Russian River in Sonoma County, north of San Francisco, was the warmest swim I had in California. I swam from one of the river beaches in Monte Rio late in the evening. Earlier there had been canoes and kayaks on the water. The far bank of the river had hill slopes thickly forested with redwoods. The water was shallow until mid-stream, and, even then, it only got to about chest height.
It was clean and clear all the way across the river.