Lake Natoma visitors looking for relief from the heat may want to reconsider swimming in the water after recent samples continue to show high levels of E. coli at the Black Miners Bar Swim Area and Nimbus Flat North Beach. For the past five weeks, E. coli levels at parts of Lake Natoma have exceeded the threshold that the Environmental Protection Agency considers safe in recreational waters. The latest samples show that Black Miners Bar Swim Area is more than five times the limit, while Nimbus Flat North Beach is more than three times the limit. … A full investigation has not been conducted at Lake Natoma to confirm the cause of the elevated E. coli levels, but [California State Water Resources Control Board spokesperson Blair] Robertson said birds could be behind the increase.