<p>Urban and peri-urban wastewater treatment systems are increasingly playing a conservation role for waterbirds. Chilean Flamingos (<i>Phoenicopterus chilensis</i>) natural breeding sites are few and all of them threatened by human activities. We present baseline data on an unreported breeding site for Chilean Flamingos, El Salitral, a semi-natural wetland located near two cities in northeastern Patagonia, Argentina. This wetland receives surplus water from a wastewater treatment system. We assessed Chilean Flamingo numbers, colony size and location, and breeding output in relation to wetland water levels based on monthly visits from September to March from 2016 to 2022, and using drone photographs and satellite images. Chilean Flamingos bred at El Salitral in all years, except in 2019. Colony size varied between 1,499 and 5,915 nests, and breeding output between 0.16 and 0.44 chicks per nest. Two different nesting scenarios were identified depending on the water level of the wetland. While a central islet emerged at low levels, only a few islets very close to the wetland margin were available at high levels, increasing flamingo vulnerability to predation and disturbance. Water level influenced the location of the colony and was correlated with the number of breeding pairs, but not with flamingo numbers during the pre-laying stage nor breeding output. We discuss the main threats faced by Chilean Flamingos at this semi-natural wetland in relation to water levels, nesting scenarios and flamingo vulnerability, and provide recommendations to improve their conservation. Effective management of water level should be integrated in future conservation plans for Chilean Flamingos at El Salitral.</p>