Klamath Lake reeks like a sewer. It's blamed on the rotting of dead blue-green algae that collects along the shoreline. I have yet to find much mention of WHY there's so much, but looking at the surrounding area's green fields laden with cattle, I suspect nitrogen run-off.
Livestock production, mostly cattle, covers the entire upper lake region with myriad small streams coursing through the pastures and draining into Klamath Lake. No boaters, definitely no swimmers, a few birds. Signs warn thoroughly gut and SKIN any fish caught to eat. Even THEN you're taking a chance. Blue-green algae poisoning results in severe nausea, vomiting, etc., etc.
But those cows and ranchers are our sacred Western Lifestyle. Too bad for the lake.