Responding to accelerated recreational pressure at Diamond Lake, Oregon, in 1969 the U.S. Forest Service began installation of a wastewater diversion system which would eventually carry 85 to 90% of the sewage out of the watershed. From 1971 through 1977 the U.S. Environmental Protection Agency conducted a program of research on the lake to determine its trophic status and identify changes that might be the result of the diversion. The lake is quite productive as the result of natural loading from tributaries, groundwater and bottom sediments. Cultural influence, initially speculated to be significant was discovered to have a relatively minor impact on the lake. Total phosphorus and chlorphyll a levels reached a low in 1973, but by 1977 had increased to levels comparable to 1971. Species composition of the benthic macroinvertebrate population was the same in 1976/1977 as it was at the beginning of the study. Recommendations include an adaptation of the Dillon and Rigler system for determining the development capacity of lakes.