Data recorded from 1973 through 1978 by monitors on West Branch Brandywine Creek at Modena, East Branch Brandywine Creek below Downingtown, and Brandywine Creek at Chadds Ford were analyzed to evaluate the effectiveness of the monitors in describing water-quality conditions. The data show that the stream at Modena and below Downingtown had periods of low dissolved oxygen, and that the stream below Downingtown had a particularly severe dissolved-oxygen deficiency from May through September 1974. The pH at all three sites exceeded 9.0 at times and the pH below Downingtown occasionally dropped below 6.0. The most critical period in the basin was from March to November when pH was most likely to exceed water-quality standards and dissolved oxygen was most likely to fall below the standards. Without the monitors, the extent and magnitude of these problems would not have been detected.