Hiker Impacts on Aquatic Invertebrate Assemblages in the North Fork of the Virgin River in Zion National Park, Utah

The effects of in-stream hiking on aquatic invertebrate assemblages were examined in the North Fork of the Virgin River in Zion National Park, Utah. A localized effects study was used to investigate hiker impacts on aquatic invertebrate drift at three sites in concentrated use areas. A broad-scale study was used to determine how anthropogenic disturbance affected benthic invertebrate assemblages and their resources in the context of natural disturbance (flash flooding) and environmental variation at 11 sites of varying visitor use levels. Total aquatic invertebrate drift densities increased with increasing hiker numbers to a threshold level when approximately 40 hikers were hiking in the stream during a 30-minute period. At a broad scale, abundance of the most common taxon (Baetis) was lower at high use sites as compared to low use sites. No differences in genera richness, total abundance, or abundance of eight other common taxa were observed among use levels. Flow events appeared to play a larger role in shaping aquatic invertebrate communities than did hikers. A flow increase caused by a side-canyon flash flood increased aquatic invertebrate abundance at three sites in July 2006, while a flash flood with a six-year return interval reduced total aquatic invertebrate abundances at all sites in August 2006. These results suggest that, while visitor use in Zion National Park affects aquatic invertebrates at a localized scale, large-scale natural disturbance can ameliorate these impacts.