Glen Canyon Dam Adaptive Management Program Administrative History

River rafting trips and hikers use sandbars along the Colorado River in Marble and Grand Canyons as campsites. The U.S. Geological Survey evaluated the effects of Glen Canyon Dam operations on campsite areas on sandbars along the Colorado River in Grand Canyon National Park. Campsite area was measured annually from 1998 to 2012 at 37 study sites between Lees Ferry and Diamond Creek, Arizona. The primary purpose of this report is to present the methods and results of the project.

Campsite area surveys were conducted using total station survey methods to outline the perimeter of camping area at each study site. Campsite area is defined as any region of smooth substrate (most commonly sand) with no more than an 8 degree slope and little or no vegetation. We used this definition, but relaxed the slope criteria to include steeper areas near boat mooring locations where campers typically establish their kitchens.

The results show that campsite area decreased over the course of the study period, but at a rate that varied by elevation zone and by survey period. Time-series plots show that from 1998 to 2012, high stage-elevation (greater than the 25,000 ft3/s stage-elevation) campsite area decreased significantly, although there was no significant trend in low stage-elevation (15,000–20,000 ft3/s) campsite area. High stage-elevation campsite area increased after the 2004 and 2008 high flows, but decreased in the intervals between high flows. Although no overall trend was detected for low stage-elevation campsite areas, they did increase after high-volume dam releases equal to or greater than about 20,000 ft3/s. We conclude that dam operations have not met the management objectives of the Glen Canyon Adaptive Management program to increase the size of camping beaches in critical and non-critical reaches of the Colorado River between Glen Canyon Dam and Lake Mead.

It is apparent that before emplacement of the dam gully degradation in terraces was restored by periodic alluvial deposition from river floods, but perhaps even more important is the redistribution of flood sands onto higher terraces by wind. Thus, we propose the term "restorative base-level hypothesis" to emphasize the dynamic equilibrium between gully erosion and renewed deposition, a process that remains active in Cataract Canyon but is disrupted in Grand Canyon by the presence and operation of the dam.

We developed type geomorphic settings to develop a conceptual process model for the diverse small-catchment geomorphic system in Grand Canyon. Research findings explain how streams are able to cross broad, flat terraces given a rainfall event and how they become progressively more integrated with the river. The primary channelization processes are ponding and overflow, alluvial fan progradation, and infiltration and piping, all of which contribute to nickpoint migration. An understanding of these processes was essential to building the geomorphic model.

The predictive mathematical model quantifies erosional vulnerability by applying a hypothetical rainfall event of 25 mm/hour onto a catchment above a "pristine" terrace sequence. The principal driving factor for erosion is basin area. The principal resisting factor for erosion is terrace diffusion capacity, which is a function of terrace sand cross-sectional area and infiltration capacity. Several important modifying factors are applied to the basic model to determine relative vulnerability of each terrace to gully erosion. Vulnerability of the top terrace at each catchment is plotted against the measured amount of gully erosion in that terrace, providing a base line against which progressive changes in gully depth can be easily monitored in the future.

Field studies and research show that: (1) gully erosion of terraces has been severe during the past 20 years in Grand Canyon due to unusually high precipitation; and (2) sediment deprivation coupled with the lack of large annual floods has caused a reduction in restorative (depositional) factors. Continued measurement and documentation of geomorphic processes in catchments, particularly at type geomorphic settings, will further refine and verify the predictability of the model. We conclude that beach-habitat-building flows are essential for initiating natural restorative processes and that one of the most important processes in gully mitigation may be eolian reworking of newly deposited flood sands onto higher terraces. Prior to the construction of Glen Canyon Dam, gully-deepening and river/wind depositional processes were in dynamic equilibrium, allowing the preservation of ancient cultural sites for the past several thousand years.

Vegetation changes in the canyon of the Colorado River between Glen Canyon Dam and Lake Mead were studied by comparing photo­ graphs taken prior to the completion of the Glen Canyon Dam in 1963 with those taken afterwards at the same sites. The old photo­ graphs, taken by J. K. Millers, T. H. O'Sullivan, William Bell, F. A. Nims, R. B. Stanton, N. W. Carkhuff, N. H. Darton, L. R. Freeman, E. C. LaRue, and others, document conditions as they were between 1872 and 1963. In general, the older pictures show an absence of riparian plants along the banks of the river. The new photographs of each pair were taken in 1972 through 1976. The most obvious vege­tation change revealed by the photograph comparison is the in­ creased density of many species. Exotic species, such as saltcedar and camelthorn, and native riparian plants, such as sandbar willow, arrowweed, desert broom, and cattail, now form a new riparian com­munity along much of the channel of the Colorado River between Glen Canyon Dam and the Grand Wash Cliffs.

The matched photographs also reveal that changes have occurred in the amount of sand and silt deposited along the banks. The photo­ graphs show that in some areas erosion has been significant since the time of the earlier photograph while at other locations sediment has accumulated on river bars and terraces. Detailed maps are presented showing distribution of 25 plant species. Some of these, such as Russian olive and elm, were unknown along the Grand Canyon reach of the Colorado River before 1976.

Relevant data are presented to show changes in the hydrologic regime since completion of Glen Canyon Dam. Flooding, as expressed by annual maximum stage, has decreased in amplitude, and its sea­ son of occurrence has changed from spring (May-June) to a longer period from April through October. Dam construction has had a moderating influence on several other hydrologic variables. Com­pared to the predam era, discharge through the year now varies within narrow limits, changing little from month to month or season to season; annual maximum discharges are now strikingly uniform, and sediment load has materially decreased. Increases have occurred in some characteristics, however, such as daily variation in river stage and median discharge.

The interaction of decreased flooding, decreased sediment load, and increased riparian plant coverage makes the future of existing river fans, bars, and terraces uncertain. The establishment of a new ecological equilibrium at the bottom of the Grand Canyon may re­ quire many decades.