Geographic Information Systems (GIS) are a valuable tool for assessing cumulative environmental impact, the incremental impact of an action when added to other past, present, and reasonable foreseeable future actions. GIS can be used to quantify rates of regional resource loss by comparing data layers representing different years. GIS can also be used to develop empirical relationships between resource loss and environmental degradation. A cumulative impact evaluation method involving aerial photointerpretation, multivariate statistical analysis, and GIS techniques was developed and used to relate past and present wetland abundance with stream water quality in the Minneapolis-St. Paul metropolitan area. The results demonstrate the importance of wetland position in the watershed to water quality, a relationship which would have been difficult to detect without the benefit of GIS assisted analysis.