The U.S. National Park Service (NPS) accommodates nearly 300 million visitors per year, visitation that has the potential to produce negative effects on fragile natural and cultural resources. The policy guidance from the NPS Management Policies recognizes the legitimacy of providing opportunities for public enjoyment of parks while acknowledging the need for managers to “seek ways to avoid, or to minimize to the greatest degree practicable, adverse impacts on park resources and values” (NPS 2001). Thus, relative to visitor use, park managers must evaluate the types and extents of resource impacts associated with recreational activities, and determine to what extent they are unacceptable and constitute impairment. Visitor impact monitoring programs can assist managers in making objective evaluations of impact acceptability and impairment and in selecting effective impact management practices by providing quantitative documentation of the types and extent of recreationrelated impacts on natural resources. Monitoring programs are explicitly authorized in Section 4.1 of the Management Policies:

Natural systems in the national park system, and the human influences upon them, will be monitored to detect change. The Service will use the results of monitoring and research to understand the detected change and to develop appropriate management actions.