Environ Monit Assess DOI 10.1007/s10661-011-1915-z Assessing possible visitor-use impacts on water quality in Yosemite National Park, California David W. Clow · Rachael S. Peavler · Jim Roche · Anna K. Panorska · James M. Thomas · Steve Smith Received: 28 March 2010 / Accepted: 27 January 2011 © Springer Science+Business Media B.V. (outside the USA) 2011 Abstract There is concern that visitor-use asso- ciated activities, such as bathing, dish washing, wastewater production, and stock animal use near lakes and streams, could cause degradation of Electronic supplementary material The online version of this article (doi:10.1007/s10661-011-1915-z) contains supplementary material, which is available to authorized users. D. W. Clow U.S. Geological Survey, Colorado Water Science Center, Denver, CO, USA D. W. Clow (B ) MS 415, Federal Center, Denver, CO 80225, USA e-mail: dwclow@usgs.gov R. S. Peavler GSI Water Solutions, Inc., Portland, OR, USA J. Roche National Park Service, Yosemite National Park, Yosemite, CA, USA A. K. Panorska Department of Mathematics, University of Nevada, Reno, NV, USA J. M. Thomas Desert Research Institute, Reno, NV, USA S. Smith U.S. Geological Survey, National Water Quality Laboratory, Denver, CO, USA water quality in Yosemite National Park. A study was conducted during 2004–2007 to assess pat- terns in nutrient and Escherichia coli (E. coli) con- centrations in the Merced and Tuolumne Rivers and characterize natural background concentra- tions of nutrients in the park. Results indicated that nutrient and E. coli concentrations were low, even compared to other undeveloped sites in the United States. A multiple linear regres- sion approach was used to model natural back- ground concentrations of nutrients, with basin characteristics as explanatory variables. Modeled nitrogen concentrations increased with eleva- tion, and modeled phosphorus concentrations in- creased with basin size. Observed concentrations (±uncertainty) were compared to modeled con- centrations (±uncertainty) to identify sites that might be impacted by point sources of nutrients, as indicated by large model residuals. Statistically significant differences in observed and modeled concentrations were observed at only a few loca- tions, indicating that most sites were representa- tive of natural background conditions. The empirical modeling approach used in this study can be used to estimate natural background conditions at any point along a study reach in areas minimally im- pacted by development, and may be useful for set- ting water-quality standards in many national parks. Keywords Standards · Visitors · User-capacity · Nutrients · E. coli · Yosemite