Nearly 70 years after Tucson’s Santa Cruz River dried up, 2.8 million gallons of reclaimed water began flowing daily; In 4 months, 37 species of dragonflies and damselflies were recorded.

For much of the 20th century, the Santa Cruz River that meanders through Tucson, Arizona, existed more as a memory than a living waterway. Groundwater pumping, drought and urban growth drained the river bed, leaving long stretches of dry sand and scattered puddles. Then, in a quiet but transformative experiment, treated wastewater—that is, recovered effluent—began flowing into the river at a rate of 2.8 million gallons per day. In just four months, scientists recorded 37 species of dragonflies and damselflies returning to the canal, a quick sign that life could return when the water did. As documented by the U.S. Fish and Wildlife Service and researchers at the University of Arizona, this resurgence is changing the way cities think about rivers, reuse and restoration in the desert.A river that disappearedThe Santa Cruz River has shaped Tucson’s history for thousands of years, supporting indigenous communities, Spanish missions, and later, Anglo settlements. But by the mid-20th century, the river was in trouble. Extensive pumping of groundwater for agriculture and a growing city lowered water tables. Droughts reduced natural flows. Upstream diversions and channeling further disconnected the river from its floodplain.In the 1960s and 1970s, long stretches of the Santa Cruz River through Tucson dried up for most of the year. What was left were isolated ponds, often polluted, with limited habitat for fish, insects or birds. For many residents, the “river” became a stream of dust crossed by bridges, not a living ecosystem.Effluents as an unexpected lifelineAs Tucson grew, so did its wastewater flow. The city’s treatment plants began producing large volumes of highly treated effluent, much of which was used for irrigation, industrial cooling, or discharged into evaporation ponds. In the 2000s and 2010s, managers and researchers began asking a different question: Could some of this water be used to restore parts of the Santa Cruz River?The idea was controversial at first. Some were concerned about water quality, public perception and the precedent of relying on effluent to sustain a river. But the water leaving Tucson’s advanced treatment facilities is heavily filtered and disinfected, often exceeding the quality of many natural surface waters in the region. Over time, pilot releases demonstrated that the river could handle the flow without creating public health risks, while providing clear ecological benefits.The Response Flows projectBuilding on those pilots, the Santa Cruz River Response Flows project formalized periodic releases of reclaimed water into the riverbed south of downtown Tucson. The U.S. Fish and Wildlife Service, the city of Tucson, the University of Arizona and other partners collaborated to: identify reaches where effluent could safely irrigate the canal; monitor water quality to ensure it meets strict standards for nutrients, pathogens and contaminants; track ecological responses, including insects, birds, amphibians and vegetation.The target flow was about 2.8 million gallons per day, a modest volume in human terms but transformative in a desert river that had been dry for decades.Dragonflies and damselflies as early indicatorsOne of the first and most surprising signs of recovery came from heaven. Dragonflies and damselflies, known collectively as odonates, are sensitive to water quality, flow permanence, and habitat structure. They are also relatively easy to study, making them excellent indicators of ecosystem health.Within four months of new flows, University of Arizona researchers recorded 37 species of dragonflies and damselflies along irrigated reaches of the Santa Cruz River. For context, many desert streams support only a handful of species, and some nearby dry streams support none. The rapid return of such a diverse odonate community suggested that:The water quality was sufficient to support aquatic larvae.The flow was stable enough for reproduction and development.Vegetation along the banks was responding, providing habitat for adults to perch and forage.A broader biodiversity reboundDragonflies and damselflies were just the beginning. As the flows continued, other signs of life appeared:Aquatic insects such as mayflies, caddis and beetles colonized the canal.Birds, including herons, kingfishers and swallows, began to use the river more intensively for foraging and nesting.Amphibians, such as frogs and toads, were observed breeding in ponds and backwaters.Riparian vegetation, including willows, cottonwoods and native grasses, began to green up along the banks.In ecological terms, the river was changing from a dry, fragmented channel to a functional corridor that connected habitats and supported food webs.Why this is important for the desert cityyesThe Santa Cruz experiment has implications far beyond Tucson. Throughout the arid West, many rivers have been drained, channeled or reduced to seasonal trickles. At the same time, cities are producing increasing volumes of treated wastewater as populations increase. Using some of that effluent to restore rivers offers a rare win-win situation. For example: Cities get beneficial use of reclaimed water beyond irrigation or industrial applications; rivers recover flows that support wildlife, recreation and cultural values; and communities gain new green spaces that improve quality of life and can help cool urban heat islands.The Santa Cruz study shows that even a relatively small, constant flow (2.8 million gallons per day) can trigger outsized ecological responses in a system that has been starved of water for decades.Address concerns and challengesUsing effluent to irrigate a river is not without challenges. Key topics include: Ensuring consistent water quality, especially during storms or treatment disruptions; manage nutrients to avoid excessive algal growth or eutrophication downstream; communicate clearly with the public about what “reclaimed water” means and how it is treated; and balance the use of effluent for river restoration with other demands, such as irrigation or groundwater recharge.Santa Cruz partners have worked to address these concerns through rigorous monitoring, transparent reporting, and adaptive management. If water quality drops, flows can be adjusted or temporarily stopped. If certain reaches show signs of nutrient overload, vegetation and flow regimes may be modified.A new identity for an old riverFor Tucson residents, the return of Santa Cruz is changing the way they view their city. Sections once avoided as dry, trash-filled washes are becoming places to hike, bird watch and connect with nature. Schools use the river as an open-air classroom. Community groups organize cleaning and restoration days.The river is not “wild” in the traditional sense; Its flows are managed, its water is treated effluent and its banks run through an urban landscape. But it is alive as it has not been in almost 70 years, and that life is visible in the flash of a dragonfly’s wings on the water.Lessons for the futureThe resurgence of the Santa Cruz River suggests a new model for urban rivers in dry regions:Treat wastewater to high standards and use some of it to maintain or restore river flows.Closely monitor ecological responses, using insects, birds and vegetation as indicators.Engage the community so that restored rivers become valuable public assets, not just infrastructure.Nearly seven decades after the river dried up, 2.8 million gallons a day of reclaimed water have returned not only the flow, but also a living, breathing ecosystem. In just four months, 37 species of dragonflies and damselflies found their way home: a small but vivid sign that, in the desert, adding water can still mean adding life.