A four month intensive study of the bacteriological quality of water in the Tulpehocken Creek basin indicates that (1) the streams locally contain high densities of bacteria indicative of fecal contamination, (2) nonpoint waste sources, particularly livestock, are the dominant influence in the excessive bacteriological-indicator counts observed, and (3) retention time of water in the proposed Blue Marsh Lake is believed sufficient to reduce bacteria densities to acceptable levels except following intense rainfall and runoff events during normally low flow periods.