Rochester is in Olmsted County, one of only four counties in Minnesota without a natural lake. Artificial lakes exist in the area, including Silver Lake, a dammed portion of the South Fork Zumbro River just below the convergence with Silver Creek near the city center. Silver Lake was once used as a cooling pond when the coal-burning power plant was operated by Rochester Public Utilities at the lake. When operational, the RPU coal plant's heated water output prevented the lake from generally freezing over during the winter months; attracting large numbers of migrating giant Canada geese. Large murders of crows are known to flock to Rochester during winter, including the Rochester cemetery downtown.