Hot temperatures have many headed straight to local waterways to cool off, but experts say high heat and a bad drought this year can be ideal conditions for harmful algae. It’s an issue that’s not always top of mind. … According to state data, popular waterways in the immediate Sacramento area are safe. People headed to the American River or Folsom Lake for the Fourth of July are in the clear. But the state labels part of Discovery Bay near San Francisco as a danger area, with a warning not to swim or touch the scum on the water.