Combined sewer overflow (CSO) also contributes pollution. In some areas of Syracuse, sewers carry both sanitary sewage and stormwater. During dry weather, these sewers carry all the sanitary sewage to Metro for treatment; however, during times of heavy rain or melting snow, the amount of water is greater than the capacity of the sewers. They overflow and discharge a combination of runoff and sanitary sewage into Onondaga Creek and Harbor Brook. Through these tributaries, the CSO eventually reaches Onondaga Lake. CSO has been generating concerns about bacteria. In the 1960s, Syracuse had ninety points where CSO could reach Onondaga Creek, Harbor Brook, or Ley Creek.