In 1993, the Sonoma County Water Agency (SCWA) completed the Russian River Demonstration Study. That study concluded that four of the Agency's five Russian River Ranney collectors are not under the direct influence of surface water and thus, are not subject to the requirements of the State Surface Water Filtration and Disinfection Treatment Rule. The fifth collector (Caisson 5), was found to be under the direct influence of surface water, primarily because of a significant number of positive coliform samples. The objective of this study was to determine the readily-measured conditions under which they occur so that SCWA operations staff can take action to prevent migration of Giardia and Cryptosporidium into the finished water. The parameters monitored to help define the conditions under which surface water influences Caisson 5 water were: coliform bacteria; turbidity; particles; river flow and level; and, caisson water level. Routine monitoring was used to gather background data for comparison with other data.