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A Sand Budget for Marble Canyon, Arizona—Implications for Long-Term Monitoring of Sand Storage Change

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A Sand Budget for Marble Canyon, Arizona—Implications for Long-Term Monitoring of Sand Storage Change A Sand Budget for Marble Canyon, Arizona—Implications for Long-Term Monitoring of Sand Storage Change ecent U.S. Geological Survey Rresearch is providing important insights into how best to monitor changes in the amount of tributary- derived sand stored on the bed of the Colorado River and in eddies in Marble Canyon, Arizona. Before the construction of Glen Canyon Dam and other dams upstream, sandbars in Glen, Marble, and Grand Canyons were replenished each year by sediment- rich floods. Sand input into the Colorado River is crucial to protecting endangered native fish, animals, and plants and cultural and recreational resources along the river in Glen Canyon National Recreation Area and Grand Canyon National Park. View of the Colorado River in Grand Canyon, Arizona, showing sandbars that form downstream from There is longstanding interest in tributary debris fans, which constrict the flow of the river and cause eddies. The view is looking upstream the condition and trend of river-related at a point approximately 217 miles downstream from Glen Canyon Dam. (Photo by Sam Jansen.) resources in and along the Colorado River in Glen Canyon National Recreation Area and Grand Canyons are maintained by occurred because releases of water from and Grand Canyon National Park, Arizona. replenishment of sand that is supplied by Lake Powell are virtually free of sediment. These resources include endangered native tributaries and redistributed by occasional The tributaries that enter the Colorado River fish, native riparian flora and fauna (plants controlled floods released from Lake downstream from the dam supply only a and animals that live along streams), Powell. The two largest sources of tributary fraction of the pre-dam sand supply (Topping riverside campsites, and many sites of sand are the Paria and Little Colorado and others, 2000), and the capacity of the cultural significance to Native Americans. Rivers, which enter the Colorado River 17 post-dam river to transport that sand greatly The physical underpinnings for many of and 79 miles downstream from the dam, exceeds this limited supply. Normal dam these resources are river-deposited sandbars respectively. The sand delivered by these operations, therefore, tend to erode, rather that occur intermittently along the banks of tributaries is temporarily stored on the than build, sandbars. more than 300 miles of the Colorado River riverbed until the water released during By experimentation, scientists have in Glen, Marble, and Grand Canyons. controlled floods transports it to sandbars learned that controlled floods, if released In the Colorado River, most sandbars along the river’s banks. from the reservoir immediately following form along the shoreline in eddies, which large inputs of sand from tributaries, are areas of recirculating, relatively low Sandbars in Grand Canyon— can build sandbars (Schmidt and Grams, velocity flow. The sandbars are typically Previous Research 2011). These sandbars are built during inundated during floods and exposed controlled floods when sand is carried at times of low stream flow. Before the Decline in the size and abundance of from the riverbed and temporarily completion of Glen Canyon Dam in 1963 sandbars since the pre-Glen Canyon Dam suspended at high concentration in the and the construction of other large dams era has been documented by analysis of old flow. The suspended sand is transported upstream, sandbars were replenished each aerial and ground-level photographs and into eddies where it is then deposited year by sediment-rich floods. Sandbars by topographic surveys that began in the in areas of low stream-flow velocity. are now smaller because Lake Powell, mid-1970s. Scientists have estimated that Sandbars enlarged by this process provide the nearly 200-mile long reservoir formed sandbar area in the upstream 100 miles of larger camping beaches for river-rafting by the dam, traps all upstream sediment Glen, Marble, and Grand Canyons was 25 trips and create backwater habitats used before it reaches the canyons downstream. percent less in 2000 than in average pre-dam by native fish. Newly deposited sandbars The sandbars that remain in Glen, Marble, years (Wright and others, 2005). This decline also provide areas for riparian vegetation U.S. Department of the Interior Fact Sheet 2013–3074 U.S. Geological Survey August 2013 to grow and are a source of windblown exposed sandbar is merely the highest part is based on repeat measurements of the sand. Windblown sand carried upslope of a much larger sandbar, most of which is underwater topography of the sand deposits from sandbars helps to cover and submerged. Thus, tracking trends in sand using an instrument called a multibeam potentially preserve some of the culturally storage requires tracking both the exposed and echosounder. This topographic method, significant archeological sites in Grand submerged parts of sandbars, as well as the for which uncertainty does not increase Canyon (Draut and Rubin, 2008). total amount of sand stored elsewhere on the for longer monitoring periods, provides a Scientists have also learned that riverbed. Monitoring the submerged parts of relatively precise portrayal of the locations controlled floods may erode sandbars sandbars in the Colorado River has required and amounts of changes in sand deposits. if the concentration of suspended sand the development of specialized methods. Repeat topographic measurements are, during a controlled flood is too low. The Two complementary methods are used however, difficult to implement over long concentration of sand during a flood is to monitor changes in sand deposits for river segments and are made infrequently. directly proportional to the amount of long segments of the Colorado River. The Flux-based measurements are ideally the riverbed covered by sand and the flux-based method for measuring changes suited to monitoring short-term changes in size of that sand. Higher concentrations in sand deposits is based on measurements sand storage, which are essential for making of suspended sand occur when the sand of total sand transport, referred to as “flux,” decisions about dam operations. Topographic is relatively fine and large amounts past measurement stations spaced 30 to 60 measurements, on the other hand, are more of the riverbed are covered by sand. miles apart. The difference in flux between appropriate for monitoring long-term trends These findings are incorporated in the stations represents the amount of sand that in the deposits, which are needed to evaluate current reservoir-release management has been deposited in or eroded from the the cumulative effect of management actions strategy for Glen Canyon Dam, which intervening river segment. Because stations over several years. Grams and others (2013) involves releasing controlled floods— operate continuously, this method provides integrated these two measurement methods administratively referred to as High a continuous record of changes in storage for the 61-mile length of Marble Canyon. Flow Experiments (HFEs)—whenever between the measurement stations. These Because mapping channel topography is time the Paria River has recently delivered continuous measurements can be compared consuming and costly, five short (less than 3 large amounts of sand to the Colorado to dam releases during the same time period mile) reaches were selected for monitoring as River. The magnitude and duration of the to better understand how dam operations a practical alternative to mapping the entire controlled floods is adjusted to transport affect sandbar resources. Disadvantages 61-mile segment. For such an approach to just the amount of sand that has recently of this method are that uncertainty be successful, topographic changes in the been delivered from the Paria River. grows in proportion to the length of the monitoring reaches must be representative of In support of the protocol to monitoring period and that it does not changes throughout the study area. Thus, the implement HFEs, the U.S. Geological provide information about how much each primary purpose of the study was to compare Survey’s Grand Canyon Monitoring and individual sand deposit changed within the two measurement methods and thereby Research Center (GCMRC) monitors the each long river segment. The alternative determine if this sampling strategy could be amount of sand supplied from the Paria method to measure change in sand deposits River and other tributaries and measures 112°W the size and distribution of sand on the Before the construction of Glen P Canyon Dam and other dams a riverbed and in eddies. Data on long-term r i Glen Canyon National upstream, sandbars in Glen, a trends in the amount of sand stored on the R Recreation Area Marble, and Grand Canyons, i v riverbed and in eddies allows scientists e Arizona, were replenished each r to evaluate whether the cumulative effect year by sediment-rich floods. To UTAH of the recently adopted HFE management provide information crucial to 37° strategy is causing a net increase or managing these sandbars, U.S. ARIZONA Glen Canyon Dam Lake Powell decrease in the size and volume of sand Geological Survey scientists are Lees Ferry deposits along the Colorado River. A monitoring changes in sandbars significant and progressive decline in and the amount of sediment sand storage would indicate that sandbars supplied to the river in Marble Canyon. This map shows the created during controlled floods are locations of the five short (less short-lived and that the total volume of than 3 mile)
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