The Verde River Watershed in north central Arizona is home to the Verde River—one of Arizona’s few remaining rivers that flows year-round. The Verde River and its watershed are also home to many rivers and streams, lakes, Tribal lands, state parks, trails, national monuments, national forests, and diverse ecosystems. There are also two major dams and reservoirs on the Verde River: Horseshoe Dam, which creates Horseshoe Reservoir, and Bartlett Dam, which creates Bartlett Reservoir.
To raise awareness about the Verde River Watershed and its importance to Arizona communities and wildlife, Audubon Southwest and The Nature Conservancy in Arizona launched a StoryMap website: Exploring the Verde River Watershed from the headwaters down to the Valley of the Sun. Within this visually-appealing website, viewers can learn about the watershed’s land area, dams and reservoirs, potential projects that could impact riverside habitat, and restoration opportunities.
Horseshoe Dam and Reservoir
The Horseshoe Reservoir not only stores water behind its dam but has also collected sediment—soil and debris that flow from the landscape and into the Verde River. Years of sediment build-up at the reservoir has reduced the amount of water Horseshoe Reservoir can hold.
To address this problem, the U.S. Bureau of Reclamation worked with the Salt River Project and stakeholders to conduct an analysis known as the Verde Reservoirs Sediment Mitigation Study in December 2021. The study evaluated how to best deal with the sediment build up, as well as explored how to protect water supplies and adapt to climate variability. Through the study, one option that was presented included raising the height of Bartlett Dam (which is downstream of Horseshoe Dam and Reservoir) which would increase the size of Bartlett Reservoir and make up for the lost water storage capacity from sediment accumulation in the upstream Horseshoe Reservoir. In this scenario, Horseshoe Dam could allow water from the Verde River to pass through it—and down to Bartlett Lake—and revert to acting more like a river, instead of a reservoir, depending on Salt River Project operations of the Verde River system. To learn more about areas identified for potential restoration within Horseshoe Reservoir, check out the Horseshoe Restoration section of the StoryMap.
Bartlett Dam and Reservoir
To address the sediment build up, the Verde Reservoirs Sediment Mitigation Study proposed two options for increasing the height of Bartlett Dam:
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Modification Scenario 1: Bartlett Dam would be raised by 101.8 feet to reach approximately 1,920 feet in total height. This would allow the new Bartlett Reservoir to store up to 628,000 acre-feet of water—351,000 acre-feet more than the existing Bartlett Reservoir can hold.
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Modification Scenario 2: Bartlett Dam would be raised 60.8 feet to reach approximately 1,895 feet in total height. This would allow the new Bartlett Reservoir to store up to 422,000 acre-feet of water—146,000 acre-feet more than the existing Bartlett Reservoir can hold.
To better understand how these modifications would affect nearby habitat (because the proposed increased dam height would create a larger reservoir footprint that would flood out existing vegetation) Audubon Southwest and The Nature Conservancy in Arizona turned to geographic information systems (GIS) tools to perform two analyses.
These analyses are included in the StoryMap for exploration and to deepen understanding of the potential impacts to habitat along the river:
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Critical Habitat Analysis: We calculated how many acres of U.S. Fish and Wildlife Service-designated critical habitats would be inundated under either dam modification scenario (based on information provided from the Verde Reservoirs Sediment Mitigation Study).
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The Southwestern Willow Flycatcher and Western Yellow-billed Cuckoo are two federally protected bird species that nest in cottonwood and willow forests along the Verde River. By analyzing the footprint of both Bartlett Reservoir expansion scenarios, we were able to determine how much critical bird habitat could be lost under the dam modification scenarios.
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Land Cover Analysis: We classified the types of habitat (land cover) in the affected area and calculated how many acres of each type of habitat would be inundated with water from a larger reservoir under both dam modification scenarios.
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To do this, we used the ArcGIS tool “Classification Wizard” to perform a Supervised Object-based Segmentation using recent aerial imagery (2023 National Agriculture Imagery Program data). We trained the software to differentiate four habitat types: cottonwood-willow habitat, mesquite-dominated habitat, tamarisk (salt cedar) stands, and other. Neighboring pixels were grouped together based on similarity in color and shape in the process known as segmentation. The object-based segmentation method that we used smooths out the groupings according to pixel values and geographic information to most closely resemble real-world features and produce habitat classification results. Once we inventoried land cover, we calculated how many acres of each habitat type would be lost under the two proposed Bartlett Dam modifications. Our supervised object-based segmentation analysis achieved an estimated 86 percent accuracy. With new Light Detection and Ranging (LiDAR) imagery data, we could further increase the accuracy of our analysis.
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Visit the Bartlett Dam and Reservoir section of our StoryMap to view the data, analyses, and interactive maps comparing the two dam modification scenarios and the amount and type of habitat that could be impacted by a larger Bartlett Reservoir.
Next Steps
Audubon Southwest and The Nature Conservancy in Arizona will remain engaged with the Verde Reservoirs Sediment Mitigation project. We recognize the need to adapt Arizona’s water infrastructure to deal with sedimentation as well as climate and precipitation variability. This project is significant because it has the potential to provide additional renewable surface water supplies to the Phoenix Metropolitan Area, boosting regional water security. It also has the potential to reduce groundwater pumping. And new renewable water supplies are especially important as our Colorado River supplies shrink.
Around 4,000 acres of habitat could be affected by a larger Bartlett Dam and Reservoir. The landscape that could be submerged stands out not only because endangered species live here, but because it is a large, contiguous riverside ecosystem—something not often found in Arizona and not easily replaced.
Impacts would be felt at Bartlett Reservoir as well as at habitats downstream on the Verde and Salt River systems, because more water would be captured behind Bartlett Dam, and less water would be released to flow downstream in wet years.
Audubon Southwest anticipates a robust examination of the potential habitat impacts and the identification of potential mitigation opportunities within the Verde River Watershed that would intentionally support the wildlife that rely on those impacted habitats. We appreciate the consideration of habitat restoration opportunities in Horseshoe Reservoir as well. Additionally, exploration of ways to reduce sediment build up in the future—such as through watershed restoration projects that reduce erosion on the landscape—are important considerations as this project advances.
While dams and reservoirs impact a river’s ability to flow naturally and greatly alter habitats, they also allow water to be stored and then released later so that communities, Tribes, farmers, businesses, and others have access to water when they need it. The Salt River Project’s stewardship of water resources includes Habitat Conservation Plans, implemented with guidance from the U.S. Fish and Wildlife Service, to protect and preserve habitats for threatened and endangered species. Most recently, in 1996, the Salt River Project raised Roosevelt Dam to increase its water storage capacity by 20 percent to improve flood control, while enhancing recreational opportunities and mitigating the impact on habitat.
Before the sediment and lost capacity of the Horseshoe Reservoir can be addressed, the National Environmental Policy Act (NEPA) process must be completed. The U.S. Bureau of Reclamation kicked off the NEPA process on July 11 by publishing a Notice of Intent to prepare an Environmental Impact Statement in the Federal Register, which set a 30-day public comment period. For more information, visit the U.S. Bureau of Reclamation’s website.
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