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5 Sediment and Geomorphology WHY SEDIMENT AND GEOMORPHOLOGY ARE IMPORTANT Sediment is an integral part of the Grand Canyon river system. Sand entering the Colorado River from the major tributaries moves through the canyon with the river and is deposited as bars. When the bars are above the water surface, they become the beaches on which the river runners camp. Occasional movements of coarse substrate (debris flows) from tributaries create the rapids that the boaters thrill to travel through. Sand and debris flows provide habitat for riparian and aquatic communities. Thus, an un- derstanding of the processes that create bars, beaches, and rapids is a necessary part of the Glen Canyon Environmental Studies (GOES). Amount of Sediment Is Not a Problem in the Grand Canyon There is adequate sediment, of which sand is the dominant constituent, entering the Colorado River below Glen Canyon Dam to maintain bars and beaches. In fact, sand would not accumulate in the canyon if flow from the dam were constant (BOR, 1993, p. 90) but would collect in the channel and in low bars, with an elevation just below the elevation of the water surface. Thus, the problem of sand in the Colorado River is not the amount of sand but rather its distribution. 70
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Sediment and Geomorphology 71 Management of Sediment Maintenance of beaches above mean water level requires some pro- cedure for raising the water level temporarily for beach-building flows. The amount of discharge (height of water in the channel) will determine the height of the beaches that are built by beach-building flows. Furthermore, beach building will be most successful when there is a large supply of sand near the upper end of the Grand Canyon. A sand budget must be maintained that shows the amount and distribution of sand. As we will discuss later in this chapter, the critical reach for sand man- agement is between the Paria and the Little Colorado rivers. Research and data collection should concentrate on that reach of river. Of secondary concern is the reach from the Little Colorado River to Phantom Ranch. Much less important in terms of sand management is the reach below Bright Angel Creek. Sand cannot be lifted onto beaches without occasional high flows (An- drews, 1991~. The amount, duration, and scheduling of high flow are ele- ments of management decisions that must be based on good scientific data plus reliable physically based models. In general, higher and longer flows create higher beaches. Beach-building flows should coincide with the presence of high amounts of sand in the reach from the Paria to the Little Colorado rivers; this sand will be lifted to beaches downstream by the high flows. At present, the sand supply from the Paria River is less well known than that of the Little Colorado River. The Paria should be closely monitored so as to reduce the uncertainty in the river's sediment transport record. A study of the relationship of accuracy to frequency of sampling should be undertaken for the Paria. Rapids Rapids in the Grand Canyon are created by debris flows from minor tributaries throughout the canyon. Sands and gravels are eroded from the debris flows, leaving behind the cobbles and boulders. The debris flows narrow the area of flow and thus create the rapids. The size of the materials removed from the debris flow deposits depends on the peak flows that pass around and over the debris. Before the closing of Glen Canyon Dam, the rapids were reworked by large annual floods and
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72 River Resource Management in the Grand Canyon by occasional massive floods. Thus, the rapids might constrict the channel, but the amount of constriction was periodically reduced by annual and occasional major floods. Without inadvertent spills, as occurred in 1983, or deliberate flood flows, Glen Canyon Dam would cause the rapids to increase in size. Without flood flows, there would be no mechanism to remove the large material in the debris flows (Cooley et al., 1977; Dolan, 1981; Kieffer, 1987~. Each time a debris flow occurs at a site of an existing rapid, the new debris will further constrict the channel. Over a period of time, rapids could become impass- able, and river runners would need to portage around them. Thus an under- standing and monitoring of the debris flows and their removal mechanism were integral parts of GCES. Sand as Substrate Sand deposits are a substrate that supports riparian and aquatic organisms in the canyon river corridor. Backwaters behind the beaches provide habitat for warmwater-adapted fishes, such as the humpback chub (Chapter 6~. Releases from Glen Canyon Dam are cold and swift, and the backwaters reduce current velocity and may also give the water a chance to warm. The bars and beaches support vascular plants and thus provide habitat for terrestrial animals. Thus, an understanding of the physics of the sand system and of the geomorphology of the canyon are important for understanding the occurrence and abundance of the flora and fauna throughout the canyon. In the past, sand has covered archeological sites and thus has protected them. Periodic erosion of the sand through local flooding was offset by deposition of new sand from the annual floods of the Colorado River. Without the high annual floods and occasional rare large floods in the Colorado River, local erosion on the beaches and bars will not be offset by deposition without some inclusion of high flows in the management plan for the dam. Local runoff now creates gullies and thus exposes ancient sites. Only flood flows can reverse this trend (Hereford et al., 1991~. SEDIMENT STUDIES OF GCES PHASE I GCES began in 1982 in response to concerns about the effects of Glen
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Sediment and Geomorphology 73 Canyon Dam on the resources of the Grand Canyon. The planners of Phase I of GCES recognized that sediment is a major resource of the Grand Canyon river corridor. The daily fluctuations of flow from Glen Canyon Dam (Chapter 4) were thought to erode the beaches and thus diminish the resources for river runners as well as the native fishes and riparian biota of the canyon. Almost as soon as GCES Phase I was organized, exceptionally high runoff occurred (in 1983~. The resulting unusual spill from the dam radically altered the canyon and its beaches in ways that were difficult to anticipate. Thus, the high flows disrupted the original plans for GCES Phase 1. Bureau of Reclamation Model GCES Phase I included studies of sediment movement through the Grand Canyon and emphasized modeling of flow and sediment transport. Modeling was undertaken by the Bureau of Reclamation (BOR), which planned to adapt a preexisting steady-state program to model the flow through the canyon (Randle and Pemberton, 1987~. The movement of flood waves was modeled by a series of steps involving different discharges. Thus, the flow was allowed to vary, but the shape of the flood wave was held constant. The BOR modeled sand movement by using the hydraulics developed by the water-routing model with calibration to sand discharge relationship at various control points in the canyon. The gauge below the Little Colorado River and the long-term gauge at Phantom Ranch (Bright Angel Creek) were to be the anchors of the model, and other temporary gauges were used to supplement the data base. There were two major flaws in the BOR approach to modeling in GCES Phase 1. First, a steacly-state model cannot predict the attenuation of a flood wave as it moves down a river. The second major flaw was that the sand discharge relationships at several measurement sites in the canyon could not be differentiated from each other. Thus, the model was necessarily calibrated to identical relationships at different sites, which led to the conclusion that the sand input and output would be the same throughout the reach of the canyon modeled. Thus, there was no basis for estimating the variation in storage of sand within reaches of the canyon. GCES Phase I monitored the beaches, their geometry, and theirvolume. There was, however, no strategy for relating these results to the water- and sediment-routing model.
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74 River Resource Management in the Grand Canyon Early Assumptions Concerning Sediment Movement GCES started with the assumption that beaches were being eroded throughout the Grand Canyon and that the cause was fluctuating flows released from Glen Canyon Dam. The judgments stated above were those of the river boatmen, but they strongly influenced the GCES researchers. These assumptions influenced the decisions concerning what should be studied, how it should be studied, and, more importantly, the conclusions from GCES Phase 1. Part of the learning experience of GCES Phase I was to set research goals as scientific hypotheses to be tested rather than ass- umptions to be verified. Coordination and Communication "Unfortunately, the critical linkage of sediment/water flow in the main channel was pursued predominantly as an exercise in its own right, largely divorced of concerns about sediment sources and sinks and with inadequate attention to modeled sediment movement to beaches, riparian habitats, and so on" (NRC, 1987, pp. 88-89)-so stated the National Research Council (NRC) committee in its assessment of the sediment work in GCES 1. Good work was performed and excellent data were collected, but there was little coordination among the different elements of the research team. Elements were added to the plan as time showed that the original plan would not provide sufficient information. The spill of 1983 radically altered the work plan but did not lead to full integration of the study team.. New projects were added to study newly perceived problems, but each project remained essentially an independent entity. There was little coordination of results and little exchange of information among research teams. The water and sand modeling was not related to the beach studies, and there was no mechanism for the results of one to be integrated into the work plan of the other. Leadership for GCES GCES I was organized around the capabilities and approaches of governmental agencies. Each agency pursued its work independently, with little sense of overview. This resulted partly from the fact that each agency was considered an expert in its field, and the GCES leadership was not
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Sediment and Geomorphology 75 sufficiently empowered to overrule the judgment of individual agencies forthe benefit of GCES (Chapter 2~. For sediment research the agencies were asked to propose projects that would address the general goals of GCES. There was no a priori statement of tasks, with work plans, and with requests for agencies to perform the work as designed by GCES. Thus, for example, the BOR worked on its water and sediment model without anyone asking how the model and its results would fit into the solution of the problems to be addressed by GCES 1. NRC Committee Recommendations After GCES ~ At the end of GCES 1, the NRC committee made several suggestions concerning future GCES work. Several suggestions were particular to sed- iment work. Others, though more general, had direct bearing on it. The particular sediment-related suggestions were summarized as follows: "Future work by the Department of the Interior should seek to look for connections between research disciplines in the planning phases of the study, initiate studies of tributary processes because they are the main source of sediment in the Colorado River main stem, include in future hydrologic research empirical approaches es well as modeling approaches, link sediment studies to biological and hydrological monitoring and research, and institute geomorphic studies to supplement the hydraulic studies of the Colorado River system in the Grand Canyon" (NRC, 1987, p. 8~. The GCES team implemented some of these suggestions. In particular, the tributary sources of sediment were given more emphasis. Both empirical and physically based modeling replaced the original modeling effort. The general problems of coordination and integration persisted. Different groups collecting similar data at a site dicl not always communicate their findings or plans to each other. There seemed to be no understanding that the data collection could be better coordinated, so that the overall results of GCES could be achieved with less effort. There seemed to be little attempt by researchers to query each other before a river trip to determine whether there were data that could be collected for general use or that could be collected in a slightly different manner or format for the benefit of others. As an example, the slopes of the bar faces were studied, but bar topography was documented separately and without acknowledgment of the values of computing bar face slopes for use by others.
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76 River Resource Management in the Grand Canyon WHAT IS NEEDED CONCERNING SEDIMENT AND GEOMORPHOLOGY Sediment Budget The perception of the problems concerning sand in the Grand Canyon changed over the life of GOES. One of the concepts that instigated GOES was the belief that fluctuating flows from the Glen Canyon Dam were causing a loss of the sand bars, which are the camping beaches on which the public depends in their trips through the canyon. It was thought that steady flows would reduce the removal of sands and improve the condition of the canyon as a whole. About 790,000 tons of sand enterthe Colorado River annuallythrough the Paria, and another 1,600,000 tons enter through the Little Colorado River. Kanab Creek contributes 300,000 tons per year, and other minor tributaries arm debris flows from side canyons contribute about 700,000 tons of Sara per year. Thus, there is on average, over 3 million tons of sand delivered per year to the river corridor (BOR, 1993, Appendix D). If the releases from Glen Canyon Dam were constant, about 11,400 cubic feet per second (cfs) would correspond to a release of 8.25 million acre-feet (ma] per year as specified by the Law of the River, and 15,200 cfs would carry the average inflow to Lake Powell (11 mat per year). A steady flow of 15,200 cfs would carry about 550,000 tons of sand per year to the Little Colorado River, and some 1.13 million tons per year past Phantom Ranch and out of the canyon into Lake Mead (BOR, 1993, Appendix D). Thus, with steady flows, sand would ac- cumulate in the canyon (almost 2 million tons per year). It would accumulate in the main channel, however, and not on the beaches and bars. Wave action eventually would cause all bars to erode to the elevation of the water if flows were steady. If steady flow persisted, more of the bottom of the channel would be covered with sand and a new equilibrium condition would developthatwould carrythe sand through the Grand Canyon with bars at water level. The only variation in this condition would be temporary, following storm flows from the Paria and Little Colorado rivers, which might deposit sand at higher elevations. Sediment Budget by Reach The problem of sand management varies in different parts of the canyon.
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Sediment and Geomorphology 77 In the first 16 miles, from Glen Canyon Dam to Lees Ferry and the mouth of the Paria River, there is almost no sand inflow (38,600 tons per year from minor tributaries). Therefore, the sand storage in that reach is declining. Sand transport past the mouth of the Paria therefore is a net loss to the reach above. This can be ameliorated only by sand augmentation, which presently seems unlikely (Chaptered. Otherwise, all sand in thefirst 16 miles eventually will move downstream, and there will be no beaches in that reach. In the next 61 miles, between the mouth of the Paria and the mouth of the Little Colorado, the only significant sand supply is from the Paria. The distribution of sand in that reach is dynamic, and the capacity of the water much exceeds that required to move the sand downstream. This is the critical reach, and the problem of sand management is to keep as much sand as possible on the beaches and out of the main channel. Below the Little Colorado River, almost the total supply of sand is available in the Colorado River. The first 26 miles, between the Little Colorado to Bright Angel Creek, will, however, have periods of beach erosion when sand supply to the Colorado River from the Little Colorado is below normal. Beach erosion should be considered normal during periods of sand supply deficit, and sand accumulation should be expected during periods of high sand transport from the Little Colorado. Any strategy that improves the reach from the Paria to the Little Colorado probably also will improve the reach just downstream from the Little Colorado. This is because the flows from the Paria and the Little Colorado are correlated; high flows and high transport of sand usually occur from both in the same year. Thus, when there is a supply in the upper reach to be managed, there usually is a supply in the reach just below the Little Colorado also. The Paria, which is much smaller than the Little Colorado (Chapter 4), may not increase the main river flow sufficiently to transport the sand that it delivers. Thus, much of the sand contributed by the Paria will tend to remain in the channel immediately below the river's mouth. In contrast, the peak flows from the Little Colorado combined with flows from the Paria and with dam releases often exceed the power plant capacity of 31,500 cfs from Glen Canyon Dam and will deposit materials on the beaches throughout the system downstream from the Little Colorado. In particular, if the Little Colorado peak coincides with high flow from Glen Canyon Dam, a sizable peak of discharge and transport can occur. The most critical piece of information in the sand budget will be the amount of sand in the main channel between the Paria and the Little Col
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78 River Resource Management in the Grand Canyon orado. There should be some decision criterion that triggers the release of beach-building flows sufficient to deposit sand from the main channel onto the beaches in the reach above the Little Colorado. The amount of sand in storage in that reach should be a part of the triggering, as well as the water stored in Luke Powell. When both are above normal, beach-building flows can and should be planned. Sand Distribution Between Main Channel and Beaches Beaches form where ecldies can remove sand from the main channel. Beaches often form below rapids, which create eddies downstream. Know- lecige of the mechanism of eddy formation, the variation of the mechanism with discharge, and the process for transfer of suspended sand across the eddy fence (a vertical plane that clivides an eddy field from other parts of the channel flow that are moving in a downstream direction) into the eddy field (an area of flowing water where motion occurs in a circular fashion or in a reverse direction to the rest of the channel) is needed. Preliminary evidence seems to indicate that the transfer of suspended sand from the main channel across the eddy fence is fairly rapid (Dawdy, personal communication, 1993~. Therefore, the bars will not be eliminated as long as there is suspended sand in the main channel. Eventually, the bar within an eddy will develop to a height about equal to the steady water level. Thus, a strategy must be developed not only for predicting the movement of sand across the eddy fence but also for predicting the deposition of sand on the beaches at higher elevations during beach-building flows. The river stage heights of the beach-building flows will determine the height of the camping beaches. The greaterthe flows, the higher the beaches will be and the greater the amount of sand that will be stored on the beaches. An understanding of the eddy system and its role in beach building is an integral part of managing the resources of the Grand Canyon. Flood Waves Peaks attenuate as they travel downstream. The attenuation can be predicted quitewell with the modelsdeveloped bythe USGS for GOES (Smith and Wiele, in press). The flood wave changes shape as it moves down- stream, and the travel time of the wave varies as it moves downstream (Figure
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Sediment and Geomorphology 79 5.1~. The leading edge of the wave steepens as the wave moves downstream, whereas the falling edge becomes less steep. As peaks attenuate and peak stages are lower, the beaches that the wave can build will be at a lower elevation. Also, brief peaks deposit less sand. An understanding of this interaction between sand deposition and beach-building flows should be developed, so that the sand resource can be managed throughout the system. U.S. Geological Survey Effort on Sediment Modeling The U.S. Geological Survey (USGS) has developed a water-routing model that can predict the transformation of the flood wave down the channel. The steepening of the wave front, flattening of the back of the wave, lowering of the peak, and increase in discharge at the trough can be modeled. Thus, for any time at a given site, the USGS model can be used to predict the elevation of water level, given a history of discharge (Smith and Wiele, in press). The linking of discharge to sand transport is more tenuous. The USGS uses a reach-averaged cross section to model the stage. It uses the percentage of bottom covered by sand as input to its sediment transport prediction. There seems to be no periodic updating of the cross section as sand is predicted to accumulate or erode. The model is calibrated to the sediment collection sites in the canyon, which are the source of information on percentage of the bottom covered by sand in the reaches upstream from each measurement site. The model is an improvement over the steady-state model of GOES 1, but it still is the weak link in the modeling needs for managing sand. USGS Effort on Eddy Modeling Eddy modeling is farther along than main channel modeling, although it was less well understood when GOES began. There has been more emphasis on this aspect of sediment modeling than on any other, and there have been more workers attacking the problem from different aspects. Also, there seems to be good communication among workers studying geo- morphic and mathematical aspects of ecidy formation, circulation, and bar building.
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80 60 - . 50 40 ° 30 - a) 20 10 O River Resource Management in the Grand Canyon D /~ , . ./ B \~ ~ o 50 100 150 200 River mile FIGURE 5.1 Travel time as a function of river mile for discharge waves during Research Flows B and D (solid line) compared to predicted travel times [dashed line with single dots (b) and dashed line Idol. The smaller-amplitude discharge wave of Research Flow B had a lower phase speed and therefore is the lower of the two solid lines. SOURCE: U.S. Geological Survey (1991~. Linking of Sediment Modeling and Eddy Modeling At the present time, there still is a need for linking the main channel water and sand discharge with the eddy system and the growth of bars. The deficiency at this time appears to be more in the main channel sand modeling than in the eddy modeling. Some first-approximation results based on the connection of the sand transport model and the eddy circulation model should be developed to predict the growth of bars. NAU Beach and Bar Studies Northern Arizona University (NAU) has an extensive program on mapping of bars (Beus et al., 1994~. There seems to be little attempt at quantitatively relating these measurements to the building of bars or to using the data for scheduling beach-building flows. Various data collection efforts should have been coordinated with each other. The types of data collected should have
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Sediment and Geomorphology 81 been determined by GCES management, so that the NAU data could have been used by other researchers. WHERE WE STAND NOW The GCES has added considerably to our understanding of the physics of sand movement in the Colorado River below Glen Canyon Dam, but there is still much that needs to be understood. Among the GCES findings related to sediment are: · There is a sufficient amount of sand reaching the Colorado River to maintain the beaches. The problem is one of management of the sand in the river. · Beach-building flows that are greaterthan normal maximum operating flows are necessary to place sand on the beaches to develop camping sites. · The transformation of the flood wave as it travels downstream from Glen Canyon Dam can be modeled, so that stage and discharge at any point and at any time along the Colorado downstream from the dam can be estimated. · The critical reach for sand budget and beach stability is that from the Paria to the Little Colorado. · Rapids in the Grand Canyon will continue to grow, with no mechanism to rework rapicis, unless controlled floods are part of the management plan. RECOMMENDATIONS GCES has improved our understanding of sand movement and storage below Glen Canyon Dam. GCES sponsored the development of a more physically based analysis of the sediment transport and the eddy circulation system of the Colorado River system. GCES also acivanced our under- standing of the bar-building system of the Colorado River. New knowledge of changes of bar area and volume over time has given more insights into the change of the Grand Canyon river corridor and bar slope failure is better understood as a result of physically based modeling. The qualitatively rapid interchange of sand between the main channel and the eddy systems has been defined. The need for beach-building flows has been presented and is generally accepted by researchers. Several kinds of continuing research and monitoring will enhance the potential for beneficial management of sediment
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82 transport: River Resource Management in the Grand Canyon · Study of the rate of sand interchange between the main channel and the eddy systems that create beaches. · Development of a mechanism for determining the initiation of beach- building flows. · Development of quantification of the magnitude and duration of beach- building flows. ~ Study of the rate at which sand is deposited on beaches during beach- building flows. · Creation of a procedure for determining sand budgets in different parts of the canyon downstream from Glen Canyon Dam. Less emphasis should be placed on collection of qualitative geomorphic data and more on the understanding of sediment transport processes, es- pecially through the use of quantitative, physically based models of the system. For example, problems in using the Sediment Transport and River Simulation (STARS) water-routing model along with sediment rating curves to determine sediment transport through the Grand Canyon might have been identified earlier if more thought had been given to what was needed and how the modeling efforts would be used to meet those needs. If the present data collection methods of the U.S. Geological Survey cannot provide adequate accuracy, support of the USGS program should be shifted to monitoring the amount of change of sand volume in the critical reaches. If the amount of sand and the change in volume in the main channel cannot be measured with accuracy, the BOR should rethink its priorities for study of sediment transport and develop an alternative method for de- termining the timing and magnitude of beach-building flows. Management should coordinate the research and monitoring, even across agency lines. There should be more communication among the work- ing groups. At the very least, each research team should know who is collecting data, where the collection will occur, and the methods that will be used. Data should be exchanged. Before going into the Grand Canyon, each team should ask whether it can collect data for other researchers.
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Sediment and Geomorphology 83 REFERENCES And rows, E.D. 1991. Sediment transport in the Colorado River basin. Pp. 54-74 in Colorado River Ecology and Dam Management. Washington, D.C.: National Academy Press. Beus, S.S., M.A. Kaplinski, J.E. Hazel, Jr, and L. Kearsley. 1994. Monitoring the Effects of Interim Flows from Glen Canyon Dam on Sand Bar Dynamics and Campsite Size in the Colorado River Corridor, Grand Can- yon National Park, Arizona. Draft Final Report, U.S. Geological Survey. Bureau of Reclamation. 1993. Operation of Glen Canyon Dam. Draft Environmental Impact Statement, U.S. Department of the Interior, Wash- ington, D.C. Cooley, M., B. Aldridge, and R. Euler. 1977. Effects of the catastrophic flood of December 1966, north rim area, eastern Grand Canyon, Arizona. U.S. Geological Survey Professional Paper 980. Dawdy, D.R. 1993. Personal communication between David Dawdy and Jonathan l~lelson at GOES research meeting, October 14, Boulder, Colo. Dolan, R., and A. Howard. 1981. Geomorphology of the Colorado River in the Grand Canyon. Journal of Geology 89:269-298. Hereford, R., H. Fairley, K. Thompson, and J. Balsom. 1991. The Effect of Regulated Flows on Erosion of Archaeologic Sites at Four Areas in Eastern Grand Canyon National Park, Arizona: A Preliminary Analysis. U.S. Geological Survey, Reston, Va. Kieffer, S.W. 1987. The Rapids and Waves of the Grand Canyon. USGS Open-File Report 87-096, U.S. Geological Survey, Reston, Va. National Research Council. 1987. River and Dam Management. Washington, D.C.: National Academy Press. Randle, T.~., and E.L. Pemberton. 1987. Results and Analysis of STARS Modeling Efforts of the Colorado River in Grand Canyon. Final Report, U.S. Bureau of Reclamation, Washington, D.C. Smith, d.D., and S. Wiele. In Press. Flow and Sediment Transport in the Colorado River Between Lake Powell and Lake Mead. Boulder, Colo.: USGS Water Resources Division.
Representative terms from entire chapter: