A newspaper investigation finds part of the American River in Sacramento contains high levels of E. coli bacteria, a sign of fecal contamination.
The Sacramento Bee says this week that E. coli was found along the lower stretch of the American, where homeless camps line the banks, residents walk their dogs, and swimmers dip into the water in the summer.
Thirteen out of 15 samples taken at a beach since June exceeded state and federal standards.
Local officials have begun a study that will test DNA of the bacteria to determine its exact source.
The newspaper says homeless encampments are proliferating, and there is a widespread lack of bathroom access in the area.
But the contamination also could be a result of geese excrement.
E. coli can sometimes sicken people, with symptoms ranging from diarrhea to respiratory distress.