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U.S. Geological Survey
Open-File Report 2008-1153
version 1.0

USGS Workshop on Scientific Aspects of a Long-Term Experimental Plan for Glen Canyon Dam, April 10–11, 2007, Flagstaff, Arizona

By the Grand Canyon Monitoring and Research Center

2008

Executive Summary

Glen Canyon Dam is located in the lower reaches of Glen Canyon National Recreation Area on the Colorado River, approximately 15 miles upriver from Grand Canyon National Park (fig. 1). In 1992, Congress passed and the President signed into law the Grand Canyon Protection Act (GCPA; title XVIII, sec. 1801–1809, of Public Law 102-575), which seeks “to protect, mitigate adverse impacts to, and improve the values for which Grand Canyon National Park and Glen Canyon National Recreation Area were established.” The Glen Canyon Dam Adaptive Management Program (GCDAMP) was implemented as a result of the 1996 Record of Decision on the Operation of Glen Canyon Dam Final Environmental Impact Statement to ensure that the primary mandate of the GCPA is met through advances in information and resources management (U.S. Department of the Interior, 1995).

On November 3, 2006, the Bureau of Reclamation (Reclamation) announced it would develop a long-term experimental plan environmental impact statement (LTEP EIS) for operational activities at Glen Canyon Dam and other management actions on the Colorado River. The purpose of the long-term experimental plan is twofold: (1) to increase the scientific understanding of the ecosystem and (2) to improve and protect important downstream resources. The proposed plan would implement a structured, longterm program of experimentation to include dam operations, potential modifications to Glen Canyon Dam intake structures, and other management actions such as removal of nonnative fish species. The development of the long-term experimental plan continues efforts begun by the GCDAMP to protect resources downstream of Glen Canyon Dam, including Grand Canyon, through adaptive management and scientific experimentation.

The LTEP EIS will rely on the extensive scientific studies that have been undertaken as part of the adaptive management program by the U.S. Geological Survey’s (USGS) Grand Canyon Monitoring and Research Center (GCMRC), one of the four research stations within the USGS Southwest Biological Science Center. On April 10 and 11, 2007, at the behest of Reclamation, the GCMRC convened a workshop with scientific experts to identify one or more scientifically credible, long-term experimental options for Reclamation to consider for the LTEP EIS that would be consistent with the purpose and need for the plan. Workshop participants included government, academic, and private scientists with broad experience in the Colorado River in Grand Canyon and regulated rivers around the world. Resource managers and GCDAMP participants were also present on the second day of the workshop.

In advance of the workshop, Reclamation and LTEP EIS cooperating agencies identified 14 core scientific questions. Workshop participants were asked to consider how proposed options would address these questions, which fall primarily into four areas: (1) conservation of endangered humpback chub (Gila cypha) and other high-priority biological resources, (2) conservation of sediment resources, (3) enhancement of recreational resources, and (4) preservation of cultural resources.

A secondary objective of the workshop was the evaluation of four long-term experimental options developed by the GCDAMP Science Planning Group (SPG) (appendix B). The flow and nonflow treatments called for in the four experimental options were an important starting point for workshop discussions.

At the beginning of the workshop, participants were provided with the final LTEP EIS scoping report prepared by Reclamation. Participants were also advised that Reclamation had committed to “make every effortÉto ensure that a new population of humpback chub is established in the mainstem or one or more of the tributaries within Grand Canyon” in the 1995 Operation of Glen Canyon Dam Final Environmental Impact Statement (U.S. Department of the Interior, 1995). This decision was consistent with the U.S. Fish and Wildlife Service’s 1995 biological opinion for Glen Canyon Dam operations that describes the establishment of a “second spawning aggregation” of endangered humpback chub as a reasonable and prudent alternative.

Before beginning their discussions, workshop participants were also briefed by GCMRC scientists on the current status and trends of downstream resources, particularly native and nonnative fish and sediment resources. The following findings were presented to the participants and provided the basis for workshop discussions:

• Nonnative fish removal efforts from 2003–06, including the removal of more than 19,000 rainbow trout (Oncorhynchus mykiss), seem to have accelerated the decline of rainbow trout in the vicinity of the confluence of the Little Colorado River, a tributary of the Colorado River in Grand Canyon. Possibly in response to the reduction of competitive and predatory nonnative species, native fish represented 60 percent of the fish community captured near the confluence of the Little Colorado River in 2006, which was up from 10 percent in 2003 (U.S. Geological Survey, unpub. data, 2007); however, natural warming caused by warmer than average releases from Glen Canyon Dam (2004?06) is a confounding factor in determining the impact of mechanical removal on native fish.
  
• Between 2001 and 2005, the number of adult humpback chub (age-4+) appears to have stabilized at approximately 5,000 (Melis and others, 2006). A summary of the analysis of the Grand Canyon humpback chub published after the workshop concludes that the adult population in 2006 was approximately 6,000 (Coggins, 2007). The number of humpback chub younger than 4 years of age appears to have reached a modern low in 1991, but in 2001, the most recent year for which data are available, population numbers have increased to the levels found during the late 1980s. There is also some indication that a few young humpback chub survived in the mainstem nearshore habitats near river mile 30, which is above the Little Colorado River, during the winter of 2005–06 (U.S. Geological Survey, unpub. data, 2007).
  
• Bluehead sucker (Catostomus discobolus) and flannelmouth sucker (Catostomus latipinnis) catch rates increased fivefold between 2003 and 2006 (Arizona Game and Fish Department, unpub. data, 2007; U.S. Geological Survey, unpub. data, 2007). The presence of multiple size classes of flannelmouth sucker indicate that this species has successfully recruited during this period.
  
• Substantial increases in total eddy-sandbar area and volume are only possible during high-flow releases following large tributary floods, which enrich sand supplies in the main channel of the Colorado River (Rubin and others, 2002; Topping and others, 2006).
  
• Sandbars created by the 2004 beach/habitat-building flows (BHBF) test increased the windborne transport of sand toward some of the archaeological sites in Grand Canyon (Draut and Rubin, 2006). Increased sand carried by the wind from restored sandbars may reduce erosion and increase the preservation potential at some sites.

Participants discussed the pros and cons of the possible flow and nonflow experimental treatments identified by the SPG. Flow treatments considered by participants included steady, fluctuating, and beach/habitat-building flows. Nonflow treatments included the installation of a temperature control device (TCD), also called a selective withdrawal structure; nonnative fish control; humpback chub translocation; and increased mainstem water temperatures. The discussions relied on the available scientific literature and professional opinion. Workshop participants reached the following conclusions:

• The single most important condition that would benefit the endangered humpback chub in the near term is warming mainstem nearshore habitats, although control of nonnative species is also very important. Given existing volumetric constraints and the current state of knowledge, the most readily available tool to stabilize the presence and persistence of nearshore habitats and, thus, enhanced warming in those environments during the summer months, is steady flows from Glen Canyon Dam.
  
• Additional BHBF tests should occur following tributary sand enrichment and be evaluated to determine whether this treatment is capable of rebuilding and maintaining sandbars in a sustainable manner or if augmentation of the downstream sand supply is needed to achieve sediment conservation goals.
  
• A TCD should be built only if it is designed to release the warmest water possible. Additionally, the TCD must have the ability to release cool water under all conditions, including when reservoir conditions are relatively low. The ability to also release cool water was thought to be necessary to control the expansion of warmwater nonnative aquatic species.
  
• Managers should continue their efforts to control nonnative fish numbers, including both warmwater and coldwater species, especially in the vicinity of the Little Colorado River, where more than 90 percent of the Grand Canyon population of humpback chub spend most of their lives.
  
• Monitoring efforts should be increased for warmwater nonnative fish because they pose a significant risk to the humpback chub population and their numbers may increase if warmer water temperatures continue. Early, consistent monitoring of warmwater species is critical to successful control. It is highly unlikely that these species can be controlled as readily as rainbow trout.
  
• The risk of catastrophic loss of Grand Canyon humpback chub would be reduced if one or more populations were reproducing and growing in the mainstem Colorado River or in another Grand Canyon tributary. The best possible alternative for a spawning location is the main channel of the Colorado River.
  
• Translocation of young humpback chub from the Little Colorado River to one or more of the tributaries the Colorado River could be beneficial for the Grand Canyon humpback chub population, providing a refuge and perhaps expanding the population. It was considered unlikely that such translocations would confound humpback chub monitoring or population modeling.

Following discussions of possible treatments, the participants developed an experimental research design that was consistent with the stated purpose and need of the LTEP EIS. The experimental design, called the environmental triggers approach, uses environmental cues to trigger new experimental treatments or management actions. The best defined of these environmental triggers is the delivery of additional sediment from tributary streams to trigger BHBF tests.

An important element of the environmental triggers approach requires the specification of desired future conditions, or measurable targets, for humpback chub, sediment conservation, archaeological sites, camping beaches, and other resources of interest to managers. Explicit desired future conditions will provide reference points for evaluating treatment effectiveness and the need to implement additional treatments or management actions. Workshop participants also recommended a comprehensive monitoring program for native and nonnative fishes. The results of monitoring would be used to trigger changes in dam operation and nonnative fish control.

From 2009 through 2012, before a TCD could be built, the participants recommended testing summer and fall steady dam releases. The environmental triggers approach includes the construction and testing of a TCD.

Because it takes at least 4 years for humpback chub to reach maturity (reviewed in U.S. Fish and Wildlife Service, 2002), treatments should be applied for 4 consecutive years to measure biological response, especially in the humpback chub population. However, shorter, isolated flow treatments, such as beach/habitat-building flows, would be likely to provide new information about sediment conservation strategies and possible benefits to other resources.

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