In 1979 the National Park Service (NPS) approved a Colorado River Management Plan (CRMP) based on the Grand Canyon Wilderness Recommendation and findings from a comprehensive research program. An amendment to an Interior Appropriations Bill in 1981 prohibited the implementation of this plan and resulted in increased public use levels and continued motorized use in proposed wilderness. In the last 20 years, the demand for Whitewater experiences has increased, especially for the self-outfitted public. Today, the NPS is challenged by users and preservationists to provide accessibility while maintaining wilderness integrity. Whitewater trips along the Colorado River through the Grand Canyon National Park are an excellent example of how increasing human use is impacting a sensitive, dynamic ecosystem and threatening to degrade the quality of experience for human visitors. Although visitation of the Colorado River has remained relatively constant since the 1989 CRMP—at 20,000 to 22,000 visitors and another 3,700 guides, researchers, and park staff traveling through the Grand Canyon each year—figure 1 shows the rapid rise in visitation since 1955. Visitors travel on over 600 commercial or privately organized river trips on a variety of watercraft powered by oars, paddles, or motors for varying duration. Most of the recreational use is concentrated in the summer months, resulting in high encounter rates and congestion at riverside attraction sites. Commercially guided operations account for over 80% of the total recreational use, of which 85% is on motorized rafts. The remaining proportion of recreational river trips are undertaken by noncommercial, self-outfitted public. Nearly 60% of the self-outfitted trips occur in the summer months, with an even proportion on use in the spring and fall. Less than 1% of these trips are motorized. Major drainages and side canyons along the 277-mile river corridor in Grand Canyon National Park provide recreational activities including white water rapids, sightseeing, hiking, and swimming. Well-known attractions and destinations are regular stops for nearly every river trip that passes through the canyon. Crowding and congestion along the river at attraction sites is often extreme and has been shown to affect the character and quality of visitor experience (e.g., Shelby et a]. ).
Sediment Transport During Three Controlled-Flood Experiments on the Colorado River Downstream from Glen Canyon Dam, with Implications for Eddy-Sandbar Deposition in Grand Canyon National Park