ROCHESTER, Minn. — Bacteria in the new Cascade Lake Park, a $4.5 million municipal project, has forced the Parks Department to advise swimmers to stay out of the water. Signs went up: “Ingesting Water May Cause Illness. Swimming Not Recommended.” The lake is an old gravel pit fed by Cascade Creek on the West Side. It had been Rochester’s only public swimming option. City parks Supervisor Ben Boldt said the rise in bacteria could have been caused by run-off, excessive heat, heavy rain, or wildlife. He acknowledged that Rochester’s perennial bane of geese could be a factor.