Biotic homogenization(BH), the process where a region loses beta diversity, is a possible outcome of human impacts.  I asked if BH occurred among macroinvertebrates in an impacted stream network.  Analysis was performed on taxonomic biomonitoring data from 38 sites in Chester County, Pennsylvania collected annually from 1970 through 1997.  Beta richness, average alpha richness, and gamma richness were calculated at 5 year intervals.  I utilized a Repeated Measures ANOVA to test for differences between intervals.  Beta richness decreased by 12.98% (p <.05) between 1970 and 1997.  I detected a fourfold increase (p < .05) in average alpha richness between 1970 and 1997 and a nearly threefold increase in gamma richness for the same interval.  Results indicate that a very high degree of BH relative to previously reported values occurred in the region.  BH is often thought to result from invasion by exotic species; however, in this case BH appears to be driven by natives re-colonizing after landscape scale water quality improvement.  Results suggest a previously unidentified BH process and highlight the importance of measuring changes in beta richness in addition to alpha richness.