Fresh water is a critical resource for both ecosystems and humans. In arid regions, water withdrawals from rivers for urban and agricultural use have resulted in dramatic losses of riparian and aquatic habitats and their resident biota. As arid-land rivers dry, cultural connections to these water bodies are also negatively affected, including those of Indigenous peoples that have stewarded these rivers for millennia. However, recent advances in wastewater treatment plant technologies have resulted in high quality treated effluent that could be useful for restoring flow and ecological function in dewatered rivers. In this Research Experiences for Teachers Sites in Biological Sciences (BIORETS) program at the University of Arizona (UA), three cohorts of teacher participants will collaborate with ecologists and municipal natural resource managers to address two primary place-based research questions. First, how does the release of effluent affect the biodiversity of aquatic and riparian species in and along the urban Santa Cruz River in southern Arizona? Second, how do ecological communities change over time in these newly flowing ecosystems as new species return to the system? Teachers will work with academic mentors at UA, as well as with members of the Tohono O?odham and Pascua Yaqui tribes, to collect and analyze data. Teachers also will develop curricula for their K-12 students to participate in ecological studies, learn about water resource conservation, and reinvigorate community connections to the river. Historically, treated effluent was generally of poor water quality and it degraded ecological conditions in rivers that received effluent. Although wastewater treatment plant upgrades have greatly improved effluent water quality, large knowledge gaps remain regarding how well effluent can restore ecosystem function and riparian and aquatic biodiversity in arid-land rivers. These gaps are important to address because there are many competing demands for effluent (e.g., agricultural or golf course irrigation) which could reduce its availability to restore flow in rivers. Additionally, numerous basic ecological processes can be studied in these novel urban ecosystems, including community assembly mechanisms such as species turnover and priority effects. In this BIORETS program at UA, teachers and researchers will quantify the diversity and community structure of multiple taxonomic groups (aquatic invertebrates, riparian birds and mammals, and wetland plants) in three newly rewetted reaches of the river over three years, and compare observed patterns with data collected concurrently from nearby dry reaches. Together, these data will elucidate mechanisms of community assembly in novel ecosystems while also providing municipal managers with real-time ecological data to inform water resource management decisions and enhance urban sustainability. The results of this research will answer important ecological questions, shape water management decisions in the Santa Cruz basin, and provide a model approach for other cities to adopt for restoring flow in dewatered rivers. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.