Glen Canyon Dam Adaptive Management Program Administrative History

Between 1999 and 2005, drought in the western United States led to a >44 m fall in the level of Lake Powell (Arizona-Utah), the nation's second-largest reservoir. River discharges to the reservoir were halved, yet the rivers still incised the tops of deltas left exposed along the rim of the reservoir by the lake-level fall. Erosion of the deltas enriched the rivers in sediment such that upon entering the reservoir they discharged plunging subaqueous gravity flows, one of which was imaged acoustically. Repeat bathymetric surveys of the reservoir show that the gravity flows overtopped rockfalls and formed small subaqueous fans, locally raising sediment accumulation rates 10–100-fold. The timing of deep-basin deposition differed regionally across the reservoir with respect to lake-level change. Total mass of sediment transferred from the lake perimeter to its bottom equates to ~22 yr of river input.

This report is an important milestone in the effort by the Secretary of the Interior to implement the Grand Canyon Protection Act of 1992 (GCPA; title XVIII, secs. 1801-1809, of Public Law 102-575), the most recent authorizing legislation for Federal efforts to protect resources downstream from Glen Canyon Dam. The chapters that follow are intended to provide decision makers and the American public with relevant scientific information about the status and recent trends of the natural, cultural, and recreational resources of those portions of Grand Canyon National Park and Glen Canyon National Recreation Area affected by Glen Canyon Dam operations. Glen Canyon Dam is one of the last major dams that was built on the Colorado River and is located just south of the Arizona-Utah border in the lower reaches of Glen Canyon National Recreation Area, approximately 15 mi (24 km) upriver from Grand Canyon National Park (fig. 1). The information presented here is a product of the Glen Canyon Dam Adaptive Management Program (GCDAMP), a federally authorized initiative to ensure that the primary mandate of the GCPA is met through advances in information and resource management. The U.S. Geological Survey`s (USGS) Grand Canyon Monitoring and Research Center (GCMRC) has responsibility for the scientific monitoring and research efforts for the program, including the preparation of reports such as this one.

The year 2005 marked the 10th anniversary of the completion of the Final Environmental Impact Statement (EIS) on the Operation of Glen Canyon Dam on the Colorado River, USA. A decade of research and monitoring provides an important milestone to evaluate the effects of dam operations on resources of concern and determine whether or not the desired outcomes are being achieved, or if they are even compatible with one another or not. A comprehensive effort was undertaken to assess the scientific state of knowledge of resources of concern, as identified in the EIS. The result was the first systematic attempt by scientists to conduct an assessment of the changing state of Colorado River ecosystem resources in Grand Canyon over a decadal timeframe. In the EIS, 30 resource attributes are listed along with predictions for how those resources would respond under the Secretary of the Interior’s 1996 Record of Decision, an operating prescription based on the preferred alternative of Modified Low-Fluctuating Flows (MLFF).

Because of a lack of data or subsequent analyses to confirm whether some predictions stated in the EIS were correct, or not, 14 or 47 percent of the outcomes, are essentially unknown. Excluding outcomes that are unclear, then the remaining predictions in the EIS were correct in 7 out of 16 outcomes, or 44 percent of the categories listed. Mixed outcomes occur in 4 out of 16, or 25 percent of the categories, and failed predictions, occur in 5 out of 16, or 31 percent of the categories. As such, less than 50 percent of the outcomes were predicted correctly, underscoring the uncertainties associated with working in a large complex system with few to no long-term data sets. Similar uncertainties are faced by all resource managers charged with ecosystem restoration globally. The acceptability of this kind of uncertainty is influenced by interpretation, societal values, agency missions and mandates, and other factors. However, failure to correctly predict the future, in and of itself, is not deleterious under the paradigm of adaptive management where large uncertainties provide opportunities for learning and adjustment through an iterative process of “learning-by- doing” (Walters and Holling, 1990). Although recent science has documented a continued decline of environmental resources of the Colorado River below Glen Canyon Dam, it has also identified options that might still be implemented by managers to achieved desired future conditions in Grand Canyon.