[Display omitted] • We developed and applied the Fuegian Water Quality Index (F_WQI). • We included in its calculation periphyton Chl- a , N-NH 4 , TP, fecal coliforms and DO. • The F_WQI responded to a gradient of organic contamination along watercourses. • Periphyton Chl- a was more sensitive than the rest of the variables. • The F_WQI proved to be useful in temperate rivers and streams. The city of Ushuaia, Argentina, has experienced a rapid and unorganized population growth with possible consequences on the dynamics and functioning of urban lotic ecosystems. Urban expansion was not largely accompanied by the development of infrastructure for the provision of services such as drinking water and sewers to all the population. Currently, only 65% of the population is connected to the main sewage system (pre-treatment) and the city does not have a secondary wastewater treatment plant yet. We evaluated the impact of wastewater discharges and non-point contamination on the water quality of the three main water courses that cross the city of Ushuaia: Pipo river (PR), Buena Esperanza stream (BES) and Arroyo Grande river (AGR). We sampled four sites per watershed from spring to autumn in two consecutive years (2018 and 2019): S1 (not urbanized at the upper section), S2 (transitional section), S3 (urbanized at the middle-low section), and S4 (urbanized, close to the outlet). We developed the Fuegian Water Quality Index (F_WQI) as a tool for environmental monitoring in forest streams which includes several indicators of organic inputs (dissolved oxygen, nitrogen-ammonium, total phosphorus, fecal coliform bacteria) and periphyton chlorophyll- a (Peri Chl- a). The variables were ranked and weighed, and the index was constructed using a mathematical formula. Urbanization negatively impacted the water quality of these ecosystems; the F_WQI showed "very bad" and "bad" water quality categories at S4 in BES and AGR, both sites impacted by wastewater discharges and large percentage of urbanized area. Likewise, in the PR watershed, with less urbanization and sewage discharges, the F_WQI indicated "very good" water quality. The F_WQI summarizes the water quality of forested watersheds in a simple numerical scale, resulting in a useful tool for monitoring studies in temperate rivers and streams. We encourage the inclusion of Peri Chl- a in water quality indices, due to its ubiquity in these ecosystems and the fact that it increased the sensitivity of the index. [ABSTRACT FROM AUTHOR]