Because there were so many gold mines in the American River's watershed, there has been concern about mercury pollution in the river, including in the lower American River. "Lower" is the part below Lake Folsom, all the way down to the confluence with the Sacramento River at Discovery Park.
I looked this up because I was told it was unsafe to raft or swim in the river. And it turns out that despite these (not unreasonable) concerns, the lower American River's mercury concentration is well within EPA standards. In another source,, even taking the upper range outliers, at five sites along the lower American River the highest reading was 18.51 ng/L, well within EPA freshwater standards of 50 ng/L. (By the way, if you look in those documents - American River, okay. Cache Creek? Not so good.)
Again, this means it's safe to swim. But fish have to live in that water all day long and some of them tend to concentrate any mercury that's there. Consequently, if you're planning on eating them, here's how the California Office of Environmental health Hazard Assessment bottom-lines it about how much you should eat, not just in terms of mercury but also PCBs:
(Broken down that way to decrease fetal or childhood exposure. Adult men's brains are a lost cause, having already been damaged beyond repair by prolonged exposure to testosterone.) So swim all you want, and eat according to these guidelines.
1. Domagalski J. Mercury and methylmercury in water and sediment of the Sacramento River Basin, California. Applied Geochemsitry 16 (2001) 1677-1691.
2. Lower American River and Lake Natoma Mercury Control Program, Straw Proposal. State Water Resources Control Board, 3 Sept 2010.