Sediment Delivery to Lake Powell and Lake Mead

By Emma Krolczyk and Jack Schmidt | September 12, 2023
Hoover and Glen Canyon Dam
Part of a series of perspectives from Utah State University students on the future of the Colorado River.

Emma is a master’s student in the Department of Geosciences at USU. Her thesis focuses on using luminescence dating to date archaeological features and associated geomorphic deposits to understand climate variations in the Wind River Basin, WY. After her master’s degree, Emma will work with the U.S. Geological Survey Luminescence Dating Laboratory with the goal of expanding the applications of the technique.

Hoover Dam & Glen Canyon Dam

The Colorado River has one of the largest natural sediment loads of any large river in the United States. In 1936, Hoover Dam was completed, thereby beginning the filling of Lake Mead and the trapping of the upstream sediment load. In 1963, Glen Canyon Dam was completed and filling of Lake Powell began, thereby trapping of a significant part of the sediment load that had been delivered to Lake Mead. These two reservoirs are the largest in the United States, and the useful lifespan of these reservoirs is controlled by sedimentation in the reservoirs. Decisions about where to store Colorado River water in the future will inevitably necessitate consideration of the on-going effects of sedimentation in each reservoir.

Map of Colorado River Region

General Principles of Reservoir Sedimentation

Large dams dramatically alter the natural sediment mass balance of a river. When a dam is closed, the sediment load, that would otherwise be transported downstream, is deposited in the reservoir. Incoming bedload and suspended sediment is primarily deposited in deltas where the influent rivers enter the reservoir. Turbidity currents in some reservoirs redistribute the delta sediment further into the reservoir, sometimes all the way to the base of the dam. Sediment deposition in reservoirs reduces storage capacity and potentially impacts water withdrawal structures at the dam. Sedimentation may also affect reservoir recreation in the upstream parts of the reservoir.. Prior to the closure of Hoover Dam and Glen Canyon Dam, approximately 91 million tons per year of fine sediment were transported by the Colorado River through the Grand Canyon [7].

KEY DEFINITIONS

Delta – landform composed of sediment deposited where the river flows into the reservoir
Dead Pool – occurs when reservoir water drops below the lowest elevation to withdraw water
Knickpoint – sharp change in river channel due to erosion

 

Water levels and sedimentation in reservoirs

Lake Mead Sedimentation

Sediment accumulation in Lake Mead was significant for 27 years until  completion of Glen Canyon Dam. Thereafter, Lake Powell trapped most of the sediment delivered from the Upper Basin, but fine sediment delivered from the Paria, Little Colorado, and Virgin Rivers still accumulates in Lake Mead today. Thus, sediment delivery to Lake Mead decreased from ~91 million tons per year to ~5 million tons per year [7, 12]. Sediment deposited in Lake Mead extends the entire length of the reservoir, and the thickest deposits (~279 ft thick) occur where the Colorado River enters the reservoir. A total of 2,720,000 acre-ft of sediment, or approximately 12% of reservoir volume, has accumulated since the completion of Hoover Dam [2].

Reservoir sedimentation in deltas can cause changes in the river landscape. During the most recent lowstand of Lake Mead, the Colorado River near Pearce Ferry has downcut into its delta, but not in the same location where the channel formerly occurred. Today, the river flows over a bedrock outcrop and forms Pearce Ferry Rapids, unnavigable at most reservoir levels and a blockage to  upstream fish migration from the reservoir.

Sediment at Lake Mead Map 

 Sediment Delivery to Lake Mead

Effects on Reservoir Capacity

Lake Mead is the largest and most important storage reservoir in the Colorado River watershed, and its releases immediately meet downstream demands. The reservoir provides 90% of southern Nevada’s water supply. Southern Nevada Water Authority has pumping stations at 1,050 ft, 1,000 ft, and 875 ft elevation. Water levels at the reservoir have already dropped below the highest pumping station. If the water level at Lake Mead continues to drop, the low reservoir level pumping station at 875 ft may have to battle between lack of water and increased sediment depth [8]. With a low modern rate of sedimentation, the estimated lifespan of Lake Mead has been extended to more than a thousand years. The reservoir reaches dead pool when the water level falls below 895 feet elevation. For Lake Mead sedimentation to fill to dead pool elevation, another ~340 feet of sediment would have to be delivered to the dam. Thanks to Lake Powell, Lake Mead will continue to be able to store water for the Lower Basin for the foreseeable future. The limiting factor for the reservoir’s lifespan is now diminishing water inflow rather than sediment accumulation.

Lake Powell Sedimentation

Sedimentation in Lake Powell reduces its useful life span at the same times as the sedimentation extends the lifespan of Lake Mead. The filling of Lake Powell during more than 50 years has resulted in deltas at the mouths of Colorado and San Juan Rivers. Between 2004-2005, the water level dropped below the upper delta plain and the river began to incise a new channel through its own delta. Estimates of the pre-dam fine sediment transport through Glen Canyon were 63 million tons per year [7]. Today, fine sediment delivery is probably somewhat less, but remains significant. Additionally, the deltas have been exposed during reservoir lows and are being reworked and redeposited further into the reservoir [3].

Sedimetnation at lake powell

Effects on Reservoir Capacity

  • The most recent bathymetric surveys report that sedimentation in Lake Powell has decreased its storage capacity by 6.8% or 1.83 million acre-feet from 1963 to 2018 [4].
  • Deadpool at Lake Powell is 3,370 feet elevation. The reservoir floor is now at ~3132 feet elevation at the dam. Another ~238 feet of sediment delivered to the dam would aggrade the bed to dead pool elevation. Estimates of the future life of the reservoir range from 80 to 500 years.
  • Some groups have advocated prioritization of water storage in Lake Mead and bypassing of Glen Canyon Dam. Any sediment that bypasses Lake Powell would be delivered to Mead, thereby initiating a new episode of accelerated sedimentation in Lake Mead.

Delta advancement to Lake Powell

These plots were modified from an investigation by the USGS Utah Water Science Center in cooperation with Utah Department of Environmental Quality, Reclamation, Bureau of Land Management, National Park Service, University of Utah, and Utah State University. These graphs were created from  several bathymetric surveys and demonstrate the advance of deltas in the Colorado and the San Juan arms between 1963 and 2018. Large runoff in 2023 is anticipated to cause additional incision during the beginning of the flood, followed by upstream deposition as the reservoir refills

Sediment Management

Methods for removing some of the sediment from large reservoirs consist of flushing, excavation, dredging, and sluicing. thereby increasing the Lake Powell’s lifespan. No economic analysis has been conducted on implementing these management strategies at Lake Powell or Lake Mead.

  • Flushing – This process is conducted by scouring reservoir sediment through low level outlets such as redrilling the sealed river diversion tunnels.
  • Excavation – This process involves moving sediment with excavators. It requires the sediment to be relatively dry.
  • Dredging – This process combines digging into the sediment and removing it by using suction or creating a vacuum. This is the most common method for sediment removal in reservoirs.
  • Sluicing – This process involves rinsing out the reservoir with a pipeline via moveable raft or permanent pipeline. This process has been used on small reservoirs.

In 2008, Reclamation investigated augmenting sediment supply in Grand Canyon by dredging and sluicing sediment from Navajo Canyon delta in Lake Powell to contribute to beach building processes.

Lake Powell Sediment Statistics

Conclusions
  • There is a large flux of sediment through the Southern Colorado Plateau. Before 1963, the sediment was trapped in Lake Mead. Today, the sediment flux is primarily trapped in Lake Powell.
  • Due to low water levels, the river has incised through previously deposited reservoir sediment, but the new location of the incising channel has created knickpoints and bedrock lips where the river is less navigable. These knickpoints limit upstream migration of reservoir fish into influent channels.
  • Diversion of the Colorado River around Lake Powell would deliver large amounts of fine sediment to Lake Mead.
  • Although Lake Powell sedimentation may temporarily extend the lifespan of Lake Mead, the large-scale problem of reservoir sedimentation on the Colorado River must eventually be confronted.
References
  1. Twichell, D.C., and Cross, V.A., 2009, Surficial geology of the floor of Lake Mead   (Arizona and Nevada) as defined by sidescan-sonar imagery, lake-floor topography, and post-impoundment sediment thickness: U.S. Geological Survey Open-File Report 2009-1150.

  2. Turner, K., Rose, M.R., Holdren, G.C., Goodbred, S.L., Twichell, D.C., 2012. Environmental Setting of Lake Mead National Recreation Area. A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave. U.S. Geological Survey. link

  3. Sedimentation in Lake Powell. Recent USGS Utah Water Science Center activities in cooperation with: Utah Department of Environmental Quality, Bureau of Reclamation, Bureau of Land Management, National Park Service, University of Utah and Utah State University. 2021. link

  4. Wegner, D., Gavan, M., 2018. The Story of Sediment in Lake Powell: A Historical Perspective. Glen Canyon Institute link

  5. 5 - Elgamal, M., Fouli, H. , 2020. Sediment removal from dam reservoirs using syphon suction action. Arabian Journal of Geosciences. v. 13(943). link

  6. Randle, T.J., Lyons, J.K., Christensen, R.J., 2007. Colorado River Ecosystem Sediment Augmentation Appraisal Engineering Report. Managing Water in the West. Bureau of Reclamation. link

  7. Topping, D. J. Rubin, D. M., and Vierra, L. E., Jr. 2000, Colorado River sediment transport 1. Natural sediment supply limitation and the influence of Glen Canyon Dam. Water Resources Research 36(2): 515-542.

  8. https://www.snwa.com/our-regional-water-system/low-lake-level-pumping-station/index.html

  9. Root, J.C., Hynek, S.A., DiViesti, D.N., and Gushue, T.M., 2019, Digital Elevation Model of Glen Canyon Prior to the Flooding of Lake Powell from Historic Topographic Surveys, Utah and Arizona: U.S. Geological Survey data release, https://doi.org/10.5066/P9368XHU.

  10. Jones, D.K., and Root, J.C., 2021, Modified topobathymetric elevation data for Lake Powell: U.S. Geological Survey data release, https://doi.org/10.5066/P9H60YCF.

  11. Lake Powell’s storage capacity updated for the first time since 1986. USGS. Communications and Publishing. National News Release. Link

  12. USGS Grand Canyon Monitoring and Research Center Discharge Sediment, and Water Quality Monitoring website, https://www.gcmrc.gov/discharge_qw_sediment/.