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U.S. Geological Survey Open-File Report 2011–1220

Summary Report of Responses of Key Resources to the 2000 Low Steady Summer Flow Experiment, along the Colorado River Downstream from Glen Canyon Dam, Arizona

By Barbara E. Ralston

Thumbnail of and link to report PDF (5.9 MB)Executive Summary

In the spring and summer of 2000, a series of steady discharges of water from Glen Canyon Dam on the Colorado River were used to evaluate the effects of aquatic habitat stability and water temperatures on native fish growth and survival, with a special focus on the endangered humpback chub (Gila cypha), downstream from the dam in Grand Canyon. The steady releases were bracketed by peak powerplant releases in late-May and early-September. The duration and volume of releases from the dam varied between spring and summer. The intent of the experimental hydrograph was to mimic predam river discharge patterns by including a high, steady discharge in the spring and a low, steady discharge in the summer. The hydrologic experiment was called the Low Steady Summer Flow (LSSF) experiment because steady discharges of 226 m3/s dominated the hydrograph for 4 months from June through September 2000.

The experimental hydrograph was developed in response to one of the U.S. Fish and Wildlife Service’s Reasonable and Prudent Alternatives (RPA) in its Biological Opinion of the Operation of Glen Canyon Dam Final Environmental Impact Statement. The RPA focused on the hypothesis that seasonally adjusted steady flows were dam operations that might benefit humpback chub more than the Record of Decision operations, known as Modified Low Fluctuating Flow (MLFF) operations. Condensed timelines between planning and implementation (2 months) of the experiment and the time required for logistics, purchasing, and contracting resulted in limited data collection during the high-release part of the experiment that occurred in spring. The LSSF experiment is the longest planned hydrograph that departed from the MLFF operations since Record of Decision operations began in 1996.

As part of the experiment, several studies focused on the responses of physical properties related to environments that young-of-year (YOY) native fish might occupy (for example, measuring mainstem and shoreline water temperature, and quantifying useable shorelines). The part of the hydrograph that included a habitat maintenance flow (a 4-day spike at a powerplant capacity of 877 m3/s) and sustained high releases in April and May (averaging 509 m3/s) resulted in sediment export to Lake Mead, the reservoir downstream from Glen Canyon Dam, which is outside the study area. Some mid-elevation sandbar building (between 566 and 877 m3/s stage elevations) occurred from existing sediment deposits rather than from sediment inputs from tributaries during the previous winter. Low releases in the summer combined with low tributary sediment inputs resulted in minor sediment accumulation in the study area. The September habitat maintenance flow reworked accumulated sediment and resulted in increases in the area of some backwaters. The mainstem water temperatures in the reach near the Little Colorado River during the LSSF experiment varied little from previous years. Mainstem water temperatures in western Grand Canyon average 17 to 20°C. During the LSSF, backwaters warmed more than other shoreline environments during the day, but most backwaters returned to mainstem water temperatures overnight. Shoreline surface water temperatures from river mile (RM) 30 to 72 varied between 9 and 28°C in the middle of the day in July. These temperatures are within the optimal temperature range for humpback chub growth and spawning, which is between 15 and 24°C. How surface water temperatures transfer to subsurface water temperatures is unknown.

Data collection associated with the response of fish to the 2000 LSSF hydrograph focused on fish growth and abundance along the Colorado River in Grand Canyon. The target resource, humpback chub and other native fishes, did not respond in a strongly positive or strongly negative manner to the LSSF hydrograph during the sampling period, which extended from June to September 2000. In 2000, the mean total length of YOY native fishes was similar to the mean length from previous years, but the abundance of YOY native fish was greater in 2000. The greatest numbers of humpback chub were near the confluence of the Colorado River with the Little Colorado River, where the largest spawning population is found. Factors directly associated with the LSSF hydrograph, geography, and the abundance of nonnative salmonids in the system before the experiment, as well as elements not affected by mainstem hydrology, may have contributed to the neutral response observed for native fish. The close proximity of the Little Colorado River to Glen Canyon Dam precluded sufficient warming of the mainstem down to the confluence with the Little Colorado River (RM 61) to reach optimal growth and spawning conditions for humpback chub, unlike shoreline surface water temperatures. The 4-day habitat maintenance flow in September interrupted persistent habitats for YOY fishes and may have confounded the results. The high abundance of salmonids in the mainstem before the experiment and predation by them may have affected the number and size of native fish that were caught. Native larval fish survival in the tributaries that is unrelated to mainstem environments and flow manipulations also can affect relative abundance observed in the mainstem. Collectively, these variables limit understanding the effects of the LSSF hydrograph on young native fish growth and survival.

The complicated hydrograph composed of steady discharges at multiple volumes that varied in duration from 4 days to 8 weeks and in magnitude from 226 to 877 m3/s presented a disruption to persistent habitat, which was the intent of the experiment. The longest uninterrupted period of persistent habitat for YOY fish was 3 months. YOY fish that entered the mainstem in mid-July (for example, humpback chub) had a shorter exposure to persistent habitat. Achieving effective high-magnitude discharges for ecological experiments is a challenge in a regulated system. The presence of a dam restricts discharge magnitude, and delivery agreements among States further restricts annual and monthly volumes releases.

A change in flow magnitude is the most common element associated with regulation, and fish appear to be sensitive to this variable. The spring discharge magnitudes during the LSSF experiment were only 25 percent greater than the average MLFFs in the 1990s and 78 percent less than the average predam spring discharge. The changes in discharge associated with the experimental hydrograph likely were too small compared to standard operations to observe a response by fish. The bulk of YOY fish enter the mainstem from tributaries in the summer months, with humpback chub YOY entering the mainstem primarily in association with monsoons that typically begin in July. Trying to affect life stages (for example, spawning and larval development) that primarily are associated with tributaries that have retained their hydrology by altering mainstem volumes may be minimally effective. Instead, developing experimental flows that can target YOY life stages directly affected by mainstem hydrology and temperatures may be more informative. In contrast to experiments involving large volume releases that can often only be of short-duration, lower volume releases may be more attainable and allow testing of hypotheses about limiting factors in endangered fish species survival in the mainstem.

Other resource responses that were measured during the LSSF experiment included seedling establishment of tamarisk (Tamarix spp.), growth of wetland species during the summer, recreation safety and perceptions, and the financial costs of the experimental hydrograph to recreational businesses and power users. The LSSF hydrograph supported tamarisk seedling establishment, as the high-sustained spring flows scoured shorelines and the habitat maintenance flow transported tamarisk seeds. The reduced summer hydrograph exposed open shorelines and resulted in a proliferation of tamarisk seedlings along the scoured shorelines. The September habitat maintenance flow reduced tamarisk seedling densities associated with later season germination; those individuals that first established in June likely persisted.

The experimental hydrograph affected recreational users and businesses, and the hydrograph increased the financial costs of power. The low-discharge part of the hydrograph, with reduced water velocity, increased travel time for whitewater rafting, reduced time spent at attraction sites, increased the availability of low-water camps, and initially increased the number of boating accidents at rapids. However, the recreational experience that includes these elements and participants’ perceptions likely were affected little by the experimental hydrograph. Financial costs to the downstream commercial rafting industry included repair and replacement of equipment damaged by exposed rocks and customer refunds associated with trip evacuations because of stranding in rapids. Commercial fishing guides in Lees Ferry lost business during the habitat maintenance flows because they could not access desired fishing locales. Lastly, Federal power users incurred increased financial costs because the experiment occurred when higher than normal daily market prices had to be paid to supplement power needs. Reallocating water delivery to other months and in the subsequent water year (12 month delivery of water delivery from October to the end of September) to accommodate the hydrograph also increased costs to power users. The timing of the 2000 LSSF experiment was coincident with the onset of a drought in the American Southwest, an energy crisis in California, and market manipulation by energy suppliers that collectively affected daily market prices for power translated to increased costs to power users.

The 2000 LSSF experiment was the first seasonally based experiment using Glen Canyon Dam releases that focused on biological resources, primarily humpback chub and other native fish. Implementing such an ecosystem-scale experiment created an opportunity to learn about resource responses and identify flaws and barriers that limit experimental success. The short amount of time available for planning and implementation and the lack of long-term monitoring were apparent flaws of the 2000 LSSF experiment. Future experiments would benefit from sufficient planning, long-term monitoring, and testable hypotheses for resource responses that can be measured and are appropriate for the duration of the experiment. Future experiments would also benefit from publishing results and findings in peer-reviewed reports and journal articles that can be summarized for stakeholder use in a timely fashion. Reports by cooperators who collected and analyzed data are the first step in the process of incorporating knowledge but not the final step. Having citable literature, which can be incorporated into future experimental efforts, is critical to building a solid, peer-reviewed basis for documenting results and furthering experimental planning and decisionmaking by resource managers.

Basin hydrology and reservoir elevations greatly affect experimental capacity in the Colorado River downstream from Glen Canyon Dam. Taking advantage of unexpected sediment inputs to the system or increased water temperatures because of reduced inflows and associated reservoir elevations can be used to advance the understanding of how manipulated flow variables benefit downstream resources. If experiments were approached opportunistically, flexibility also would need to extend to administrative tasks associated with launch schedules, collection permits, and use of motorized equipment.

Experimental flexibility necessitates the implementation of long-term monitoring that provides a consistent data stream for long-term resource response. Immediate measures of response may be meaningless in the longer term, particularly for long-lived species, if consistent monitoring is absent after the experiment. A lack of response observed for 1 year may not mean the treatment was ineffective. Multiple years of data collection may be necessary for a response to be measurable or understood.

  • This report is available only on the Web.

For additional information contact:
SBSC Staff, Southwest Biological Science Center
U.S. Geological Survey
2255 N. Gemini Drive
Flagstaff, AZ 86001

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Suggested citation:

Ralston, B.E., 2011, Summary report of responses of key resources to the 2000 Low Steady Summer Flow experiment, along the Colorado River downstream from Glen Canyon Dam, Arizona: U.S. Geological Survey Open-File Report 2011-1220, 129 p.


Executive Summary

Chapter 1. Introduction and Background

Chapter 2. Physical Resource Response to the Low Steady Summer Flow Experiment

Chapter 3. Native and Nonnative Fish Response to the Steady Summer Flow Experiment in the Mainstem Downstream from Lees Ferry

Chapter 4. Vegetation Response to Low Steady Summer Flow Experiment

Chapter 5. Effects of the Low Steady Summer Flow Experiment on Campsite Area, Rafting Safety and Travel Time, and Overall Recreational Experience

Chapter 6. Effect of the Low Steady Summer Flow Experiment on Angling Quality in the Lees Ferry Trout Fishery

Chapter 7. Financial Costs Associated with the Low Summer Steady Flow Experiment

Chapter 8. Management Implications Associated with the Low Steady Summer Flow Experiment

Appendix; Annotated Bibliography of Studies Conducted During 2000 Low Steady Summer Flow Experiment

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