Utah’s Water Storage
More than 95% of Utah’s water comes from snowpack that melts each spring and runs off through watersheds into lakes and reservoirs (Figure 1; Julander & Clayton, 2018). A watershed is an area of land that drains rainfall and snowmelt into rivers and streams that flow to a common outlet such as a reservoir (U.S. Geological Survey, 2019a). A reservoir is an artificial lake built by damming a river for the purpose of water storage (National Geographic Society, n.d.). While dams and reservoirs negatively impact ecosystems by altering natural flow regimes and degrading habitat, they also provide key benefits: water stored in reservoirs can be accessed throughout the year for agricultural, domestic, and industrial uses (Poff et al., 1997; Nilsson & Berggren, 2000). Reservoir storage is especially vital during the summer when Utah receives very little precipitation, but demands for water are high (Cohen et al., 2020).
Data sources: Birch & Lutz, 2023; David et al., 2023; U.S. Geological Survey, n.d.
Figure 1. Utah’s Reservoirs and Wildfires, 1984–2021
Utah has about 3,000 natural and artificial lakes. Of these, 144 are reservoirs, and 47 are the state’s main storage reservoirs with a maximum combined storage capacity of roughly 32 million acre-feet or 10 trillion gallons of water (State of Utah, 2022; Utah Division of Water Resources, n.d.). Reservoir levels vary year to year, depending on supply and demand. Typically, peak reservoir levels occur in spring, coinciding with runoff from annual snowmelt, and minimum levels occur in late fall after water from reservoirs has been used during the summer months (Utah Division of Water Resources, n.d.). Over the past 3 years, Utah’s reservoir levels on October 1 were lowest in 2022 (42% of capacity) and higher in 2023 and 2024 (>70% of capacity; Natural Resources Conservation Service [NRCS], 2023; NRCS, 2024).
Reservoir storage levels are impacted by drought. For most of the last 20 years, Utah has experienced some level of drought, ranging from minor to extreme (U.S. Drought Monitor, n.d.). Long-term increases in Utah’s average temperatures have also decreased Utah’s peak annual snowpack by 16% from 1979 to 2023 (Hotaling & Becker, 2024). This is concerning because when less water accumulates in snowpack, less water is available to fill reservoirs in the spring, and this reduces the amount of water Utah has available for later use. And, as drought conditions worsen, the state relies more heavily on its reservoirs, which can result in withdrawals that exceed annual inputs. As climate change continues to reduce Utah’s annual snowpack, water stored in reservoirs from rainfall and years with more snow will become increasingly important (Cohen et al., 2020). Thus, maintaining Utah’s reservoir storage capacity is a key priority to protect Utah’s residents and economy. One key to maintaining capacity is minimizing the accumulation of sediment because reservoirs with more accumulated sediment cannot store as much water (Figure 2; Schleiss et al., 2016)
Sedimentation Decreases Storage Capacity
Sediment includes materials such as sand, silt, and clay that are transported through rivers and eventually settle at the bottoms of oceans, lakes, and reservoirs. Sedimentation refers to the process of sediment movement and deposition. Sedimentation rates depend on the geology, topography, and climate of an area (Moody & Martin, 2004). When fast-moving, sediment-laden water enters a
Figure 2 Impacts of Wildfire on Reservoir Sedimentation
reservoir, it slows down, and the sediment falls to the bottom. There, sediment accumulates, taking up valuable space where water could be stored (Figure 2; Schleiss et al., 2016). While sedimentation is a natural process, over time it can completely fill a reservoir, rendering it useless for water storage (Randle et al., 2021). For example, the San Clemente Dam in California, built in 1921, had lost 95% of its storage capacity by the 1990s due to sedimentation. Considered structurally unsafe and no longer useful for water storage, the dam was removed in 2015, and $83 million was spent to restore the river (Randle et al., 2021).
Typically, reservoirs store water and release it as needed through dams. However, there are almost no dams in the United States that were designed to deal with long-term sedimentation (Randle et al., 2021). Most dams were built under the “design-life paradigm,” which entails dedicating a portion of the reservoir's storage capacity to sediment accumulation. When a reservoir reaches its design-life capacity, typically about 100 years, the accumulated sediment interferes with dam outlets, and the dam is taken out of service (Wieland, 2010). This approach, however, is problematic in the long term, in part, because the number of sites suitable for reservoirs is limited (Randle et al., 2021). Moreover, dams are expensive to build and their negative impacts on fisheries and river ecosystems are wellknown, which limits the appeal of building new dams (Nilsson & Berggren, 2000; Poff et al., 1997). For these reasons, maintaining Utah’s existing reservoirs and extending their service timelines is important (Murphy et al., 2018).
Wildfires Increase Sedimentation
Managing Utah’s reservoirs amidst continued wildfire is key. Wildfires are a natural, inevitable, and, in many ways, beneficial process in the region (Pausas & Keeley, 2019). For instance, wildfires:
• Maintain open spaces for grazing and hunting
• Burn fuel so catastrophic wildfires are less likely
• Reduce vegetation so water resources are less strained
• Cause debris flows that can provide fish with desirable gravel, sediment, and nutrients
• Stimulate flowering plants that benefit pollinators
• Improve habitat complexity by creating dead trees and logs.
However, when wildfires occur, erosion and runoff can increase sediment movement by up to 1,400 times background levels, and much of this newly mobilized sediment settles in reservoirs (Smith et al., 2011). The amount of sediment deposited into a reservoir after a wildfire depends on many factors, including the size and severity of the wildfire, the surface material and topography of the watershed it occurs in, and the intensity of subsequent rainfall events (Bladon et al., 2014; Murphy et al., 2019; Smith et al., 2011). When wildfires burn, they can leave behind a waxy, water-resistant layer known as hydrophobic soil (Neary, 2004). Hydrophobic soil repels water and can increase runoff, adding to already elevated erosion in watersheds where fire has burned vegetation that normally slows water flow during heavy rains (Bladon et al., 2014).
In the western United States, wildfires are predicted to cause more intense reservoir sedimentation than anywhere else in the country due to the region’s steep topography (Moody & Martin, 2004). Utah’s steep terrain and watersheds that drain from forested areas put the state especially at risk for wildfire-induced sedimentation into reservoirs (Moody & Martin, 2004). Sediment and debris deposited into reservoirs during post-fire floods can interfere with hydropower turbines, block irrigation canals, and damage pump stations (Randle et al., 2021). Blockage of dam outlets can delay or prevent water deliveries to agricultural and domestic users (Randle et al., 2021).
Wildfires have already caused issues for water supplies to major cities, such as Denver. The 2002 Hayman Fire, for example, burned over 135,000 acres in the Colorado Rockies and severely
impacted Denver’s water supply reservoirs (Bladon et al., 2014). Despite $7.3 million spent after the fire on restoration, a $30 million dredging project was required in 2010 8 years after the fire to remove 480,000 cubic meters of sediment from the Strontia Springs Reservoir (Bladon et al., 2014).
Post-Fire Sedimentation Affects Water Quality
In addition to reducing reservoir capacity and damaging infrastructure, post-fire sedimentation in reservoirs can degrade water quality (Bladon et al., 2014). Water quality concerns range from issues with taste and appearance to toxicity levels that can cause cancer (Smith et al., 2011). Fires can release trace elements such as aluminum, arsenic, iron, and lead, which, via subsequent rainfall events and erosion, can be transported into reservoirs (Bladon et al., 2014). In addition, large postfire inputs to streams of nitrogen and phosphorus (common elements of fertilizers) can spur growth of algae and aquatic plants, reducing the amount of oxygen in the water and potentially killing fish and other organisms (Bladon et al., 2014; U.S. Geological Survey, 2019b). The effects of wildfire on water quality depend on the location of the fire, its size and severity, and the existing capacity for water treatment in that area (Smith et al., 2011). As warmer temperatures and less snowpack reduce Utah’s water supply, maintaining water quality will become even more critical (Hotaling & Becker, 2024).
Area burned (acres)
0
Year
Data source: Birch & Lutz, 2023
More Wildfires Are Burning in Utah
In Utah, preparing for sedimentation in reservoirs is a pressing issue because wildfires are becoming more widespread. Between 1984 and 2022, over 5.4 million acres (10.1% of the state’s area) were burned by wildfires (Figure 3; Birch & Lutz, 2023). Large fires have become increasingly common in recent decades, with Utah’s largest documented fire occurring in 2007 and burning over 300,000 acres (McGinty & McGinty, 2009). Still, both the area burned each year in the western U.S. and the associated post-fire sedimentation in rivers remain well below historical levels (i.e., pre-1900; Murphy et al., 2018). Concerns about wildfires are amplified now because (1) additional fuel has accumulated due to fire suppression over the past century, (2) today’s fuel is drier due to less snow and hotter summers from climate change, and (3) Utah’s population has grown, with thousands of people living in high fire-risk zones and requiring more water year-round (Gergel et al., 2017; Murphy et al., 2018). As wildfires continue increasing in size and severity due to warmer temperatures and drier fuel, postfire sediment deposition will increasingly affect Utah, given the state’s heavy reliance on water storage reservoirs (Hotaling & Becker, 2024; Murphy et al., 2018).
Figure 3. Acres Burned Annually by Wildfires in Utah, 1984–2022
Management Strategies
Multiple management approaches can help shift existing reservoirs from a fixed timeline of use to a longer-term, sustainable timeline of use (Randle et al., 2021). These strategies include:
1. Conserve water so less is needed to meet demands. This is an economical and effective step toward sustainable use because it lowers the overall pressure on reservoir storage.
2. Apply managed and low-risk prescribed burns to fire-prone forests in watersheds that drain into reservoirs. Prescribed fires can reduce fuel and, in turn, reduce wildfire severity (McGinty & McGinty, 2009) Since fire size and severity affect sedimentation rates, preventing large, highseverity fires can help maintain low sedimentation rates and protect water quality (Murphy et al., 2018).
3. Build bypass tunnels or alternate routes through or around reservoirs. This can divert sedimentation away from the water storage area (Randle et al., 2021).
4. Reclaim reservoir storage capacity by removing sediment through dredging or drawdown flushing. Drawdown flushing involves scouring and re-suspending sediment in the water column, allowing it to be released from the reservoir and transported downstream (Randle et al., 2021).
5. Raise the height of dams, which could maintain or increase storage capacity, though this comes with the risk of further degrading aquatic and terrestrial habitat (Nilsson & Berggren, 2000; Poff et al., 1997)
Looking ahead, identifying which watersheds are most vital to maintaining Utah’s water supply, are most threatened by wildfires, and are most prone to high levels of post-fire erosion and sedimentation will improve understanding about how to safeguard the region’s water supply and prepare Utah’s headwaters for growing wildfire risk.
Acknowledgments
This publication was produced as part of the Climate Adaptation Intern Program (CAIP) at Utah State University. CAIP was supported by the “Secure Water Future” project, funded by an Agriculture and Food Research Initiative Competitive Grant (#2021-69012-35916) from the USDA National Institute of Food and Agriculture, as well as support from the USGS Southwest Climate Adaptation Science Center, USU Extension, and the USU Extension Water Initiative. We improved this fact sheet based on feedback from USU Extension’s CAIP participants.
For correspondence, contact Patrick Belmont: patrick.belmont@usu.edu.
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