Response to Flows in the Santa Cruz River

Introduction

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The Santa Cruz River originates in southern Arizona, where it flows south across the Mexican border before curving northward and back into the United States. The city of Tucson developed in the floodplain of the Santa Cruz River and is now a major urban center. The Heritage Reach (‘the reach’) of the Santa Cruz, named for its historical and cultural significance to the region, runs through downtown Tucson.

Historically, the Santa Cruz River flowed perennially in several sections and served as important habitat for multiple species that are now threatened, endangered, or extinct. Historical fish assemblages in the Heritage Reach included Longfin Dace (Agosia chrysogaster), Santa Cruz Pupfish (Cyprinodon arcuatus, now extinct), Gila Chub (Gila intermedia), and Gila Topminnow (Poeciliopsis occidentalis). Additionally, lowland leopard frog (Rana yavapaiensis), Northern Mexican gartersnake (Thamnophis eques megalops), Huachuca water umbel (Lilaeopsis schaffneriana var. recurva), and California floating mussel (Anodonta californiensis) all occurred in the Santa Cruz River basin, and are now threatened, endangered, or in decline in the region. 

In 2013, the Pima County Regional Wastewater Reclamation Department constructed the Agua Nueva Water Reclamation Facility to replace former treatment facilities. The Agua Nueva facility produces effluent that is safe for all uses excluding body-contact recreation and consumption. In 2016, the city of Tucson’s water management agency, Tucson Water, began discussing restoring flow to the Heritage Reach of the Santa Cruz River through the use of reclaimed water from the Agua Nueva treatment facility, giving rise to the Heritage Project. Tucson Water set out to recharge the downtown aquifer, earn long term storage credits (LTSCs), and restore the cultural, social, and economic value of the Santa Cruz River. 

In 2019, Tucson Water started releasing treated effluent into the Heritage Reach to reintroduce river flows. Researchers at the University of Arizona began to monitor the reach following a rapid positive ecological response to rewatering. Restored flows in the Heritage Reach have provided novel habitat for native species that quickly recolonized, or were reintroduced to, the previously dewatered urban river section. 

Key Issues Addressed

The Heritage Reach flowed perennially before the 1900s. Groundwater depletion, diversions, and arroyo downcutting (erosive alteration of streambeds into deep, channelized incisions) in the late 1880s caused the river to lose perennial flow by 1913. By 1940 the river and its shallow aquifer were dewatered, and it has flowed only during flood events since.

The Santa Cruz historically supported a rich riparian riparian
Definition of riparian habitat or riparian areas.

Learn more about riparian corridor of cottonwood, mesquite, and willow woodlands along the river’s banks. This corridor provided habitat for hundreds of species of migratory birds, aquatic invertebrates, amphibians, reptiles, fish, and other wildlife. This habitat was lost as flows diminished and Tucson was built in the historic floodplain. Loss of flows directly led to the extinction of at least one endemic species, the Santa Cruz Pupfish, and contributed to the decline of other native species, including endangered Gila Topminnow. Dewatering of the Santa Cruz River further led to the loss of economic and recreational opportunities and important cultural resources.

Increasing urbanization along the Santa Cruz River basin has lowered the water table and added to habitat loss and fragmentation in the region, leading to reduced biodiversity. As drought frequency, duration, and severity increase due to climate change climate change
Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.

Learn more about climate change , the compounding loss of habitat is likely to adversely impact native plants and wildlife. Additionally, as climate change reduces water availability, demand for groundwater is likely to increase. Therefore, managed urban aquatic ecosystems are likely to play an increasingly important role in the conservation of native species and in storing groundwater for future use in the American Southwest. However, the potential benefits of, and management best practices for, highly modified urban aquatic ecosystems are not yet well understood. 

The Heritage Project is an adaptively managed urban ecosystem that provides drought-resistant habitat for native species and serves as an example for future rewatering projects in the Southwest. 

Project Goals

• Use existing infrastructure and reclaimed water to restore flows in the previously dewatered Heritage Reach of the Santa Cruz River: Tucson Water sought to recharge the local aquifer through in-stream water allocations, earn long-term storage credits, and increase the social, economic, and cultural value of the river.
• Monitor, record, and adaptively manage the Heritage Reach in order to inform ecological knowledge of urban rewatering projects and provide a framework for developing management recommendations for future urban rewatering initiatives: Researchers working closely with Tucson Water set out to use the Heritage Project as an opportunity to study the ecological benefits of the use of reclaimed water to restore flows in an increasingly urban and arid environment.
• Provide habitat for species of conservation concern: Managers at Arizona Game and Fish Department (AZGFD) and US Fish and Wildlife Service (USFWS) set out to use the new habitat afforded by the Heritage Project for the conservation of Gila Topminnow.
• Improve habitat availability for native species to mitigate the impacts of increasing regional drought: The Heritage Reach is a highly modified and managed ecosystem consisting of a new assemblage of species and interactions. Managers quickly recognized the potential for this establishing ecosystem to provide novel habitat for native species. To meet conservation goals, water managers sought to build upon relationships with local conservation organizations including Tucson Audubon Society and the Sonoran Institute as well as state and federal wildlife agencies. 

Project Highlights

Don’t Dread Dredging: In 2020, managers and researchers constructed and maintained an artificial wetland to help aquatic species return to the Heritage Reach after sediment dredging took place to reduce flood risks.

• Recharge Credits for Future Use: As part of Arizona’s 2019 Drought Contingency Plan new policy was instituted which increased previous long-term storage credit rates for effluent from 50% to 95%, meaning that now 95% of the total volume of water recharged by Tucson Water through the Heritage Project will be available for future use during periods of high demand.
• Ecological Response and Monitoring: Shortly after flows were returned, researchers at the University of Arizona began monitoring the response of vegetation, birds, invertebrates, and other wildlife in the reach through visual surveys, camera traps, and citizen science initiatives. Within three hours of the initial release of water, seven species of dragonfly and damselfly were observed in the Heritage Reach. Four months after rewatering, 37 species of dragonfly and damselfly were present in the reach, representing 75% of the known species present in lower sections of the Santa Cruz River. Additionally, three toad species and one snake were observed shortly after rewatering, and species diversity in the reach continues to increase with time.
• Topminnow Reintroductions: Gila Topminnow collected from southern sections of the Santa Cruz River were reintroduced into the Heritage Reach in October 2020, following the enrollment of the site into the Safe Harbor Agreement through a Certificate of Inclusion. Managers at Arizona Game and Fish Department and the US Fish and Wildlife Service quickly recognized the rewatered reach as a resource for conservation of species of concern, and worked with Tucson Water to enroll the site into the Safe Harbor Agreement.
• Drought-Resistant Habitat: Because the Heritage Reach is now fed by effluent and is no longer dependent on floods, it will likely not be adversely affected by future drought. Managers at Tucson Water have identified maintenance of flows in the reach as a high priority, especially following the reintroduction of Gila Topminnow. For this reason, flows are expected to be maintained during periods of drought. Because water use in the City of Tucson remains constant during drought periods, ample effluent is expected to be provided in order to maintain flows. 

Lessons Learned

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Following the restoration of flows to the Heritage Reach, diverse native taxa quickly returned to the reach. Managers suspect that the reach was particularly attractive habitat for native species because rewatering occurred in a drought year, and nearby riparian habitat is sparse. Although some revegetation efforts have been made in the reach (approximately 30 individual plants were planted by Harris Environmental, a biological consulting group, under contract with Tucson Water), vegetation communities along the river are still primarily composed of species that disperse readily. Nonnative vegetation including Tamarix spp., bermuda grass, buffelgrass (Cenchrus ciliaris), and Johnson grass (Sorghum halepense), and nonnative aquatic species including western mosquitofish (Gambusia affinis) and American bullfrog (Lithobates catesbeinas) pose ongoing management challenges in the Heritage Reach. 

Managers of the Heritage Project found that inclusion of the public was integral to its success. Tucson Water hosted public hearings and met with private landowners to conduct water quality surveys in order to address concerns about the water’s safety and increase awareness of the project. Efforts to increase public engagement in the Heritage Project increased after a large turnout at the rewatering ceremony. Social media has since played an important role in facilitating public participation. Now, volunteer efforts to clean up the basin and remove invasive species invasive species
An invasive species is any plant or animal that has spread or been introduced into a new area where they are, or could, cause harm to the environment, economy, or human, animal, or plant health. Their unwelcome presence can destroy ecosystems and cost millions of dollars.

Learn more about invasive species occur commonly under the facilitation of local conservation groups. 

The Heritage Project now includes multiple agencies, organizations, and academic institutions. Managers found that establishing a list of potential contributors early on in the project was key to its success. Leveraging existing relationships allowed for quick progress when developing conservation programs like the enrollment of the site into the Gila Topminnow Safe Harbor Agreement. Identifying and communicating achievable short-term and long-term (>5 years) goals and vision planning across time scales were important steps to securing project success. 

Ongoing communication between management and research partners and the public has allowed effective adaptive management of the reach. Managers of the Heritage Project suggest that the modified floodplain, urbanized surrounding environment, restricted potential for natural colonization, and limitations caused by flood control prevent the Heritage Reach from being fully restored to a historical stable community. Instead, they suggest that the reach operates as an “urban garden,'' wherein the Tucson community is heavily encouraged to take an active role in managing the reach through stewardship and monitoring.

Next Steps

• Continue to recharge the downtown aquifer: Tucson Water aims to continue to recharge the downtown aquifer via the Heritage Project and other managed and constructed recharge projects. Through these recharge projects, Tucson Water will provide future reserves of water and earn long-term storage credits which allow them to provide this stored water to their constituents in the future.
• Continue ecological monitoring of the Heritage Reach: Monitoring will include monthly plant, aquatic insect, bird, and wildlife surveys. AZGFD will monitor the success and viability of the reintroduced Gila Topminnow population annually, and make management recommendations according to their findings. Monitoring may also be used to develop best practices for rewatering and management of urban rivers in the Southwest.
• Reintroduce native species: Tucson water will continue to consider active restoration of native vegetation that will not compromise flood risk mitigation efforts in the river basin.
• Develop lists of historic flora and fauna: Researchers at the University of Arizona are working on a formal reconstruction of the native flora and fauna of the Santa Cruz River in the downtown Tucson region. Historical reconstructions will better inform future restoration efforts in the Santa Cruz River basin.
• Continue public engagement and outreach: Tucson Water, University of Arizona researchers, and AZGFD are continually working to develop stronger public engagement with the Heritage Project through signage discouraging the release of nonnative species, social media coverage of the river’s recovery, and partnership building with local organizations. 

Funding Partner

• City of Tucson, Tucson Water

Resources

• Tucson Water Heritage Project
• Santa Cruz Valley National Heritage Site Webpage
• Video: Reviving the Santa Cruz with Dr. Michael Bogan
• Santa Cruz River Heritage Project Birds: What We’ve Learned in a Year
• The Many Benefits of Effluent Flows in the Santa Cruz River

Contacts

• Michael T. Bogan, University of Arizona: mbogan@email.arizona.edu
• Beth Scully, Tucson Water: beth.scully@tucsonaz.gov
• Betsy Grube, Topminnow and Pupfish Aquatic Wildlife Specialist, Arizona Game and Fish Department: egrube@azgfd.gov

Case Study Lead Author

• Nicolas Katz, CART Student Intern, University of Arizona: nkatz@email.arizona.edu

Suggested Citation

Katz, N.A., Teyechea, M. (2021). “Ecological Response to Restored Flows in the Urban Santa Cruz River.” CART. Retrieved from https://www.fws.gov/project/response-flows-santa-cruz-river.