Predicting and Managing Cumulative Watershed Effects

Nonpoint sources of pollution are an increasingly important cause of water quality impairment, and this is forcing water quality regulation towards a more integrated, watershed-scale approach. Nonpoint sources are the dominant source of pollution in forested areas, and large landowners, such as the U.S. Forest Service, are required to assess and minimize adverse cumulative effects on aquatic resources. This paper argues that the current emphasis on changes in runoff is largely unjustified, whereas forest management activities can have relatively large effects on erosion and sediment yields. Although the relative importance of sediment is recognized, current models must be revised to account for the potentially larger, but less persistent, changes in erosion and sediment loads. Geographic information systems can improve our ability to assess cumulative watershed effects (CWEs). However, the accuracy of any assessment will be limited by: (1) the uncertainty of quantifying the effect of each disturbance; (2) the inability to predict indirect effects; (3) unknown recovery rates; (4) problems of validation; and (5) the uncertainty of future events. Adaptive mangement is one alternative approach, but this is unlikely to provide a sufficient level of protection to aquatic resources. Minimizing the local effect of each activity is probably the most efficient means to reduce CWEs, but this should be coupled with focussed downstream monitoring. The assessment of cumulative effects will continue to be a difficult and contentious issue.