Reservoirs regulated by small dams have a similar warming effect than lakes on the summer thermal regime of streams.

Small dams account for the majority of reservoirs throughout the world, yet little is known about their effects on stream temperatures. Given that water temperature is vital for maintaining the integrity of aquatic ecosystems, studying the effects of small dams is important. This study aims to understand the effect of small dams on summer stream temperatures in a protected area in southern Quebec, Canada. We assessed the effect of small surface-release dams on four attributes of the thermal regimes (magnitude, frequency and duration of warm events, and rate of change) of streams by comparing water temperature measured in the main tributary upstream and in the main outlet downstream of the reservoir. We also compared the thermal effects of reservoirs to those of natural lakes of similar size. Using a generalized additive model, we identified key determinants of stream temperature to assess the influence of reservoir and natural lake characteristics on the thermal regime of streams. In August 2020, we observed an average warming of 3.7 °C downstream of reservoirs regulated by small dams compared to conditions upstream of the reservoir. During this period, the warming effect of reservoirs was not significantly different from the warming effect of natural lakes (3.4 °C). In addition to the drainage area, distance to an upstream water body, and the proportion of the watershed occupied by water bodies were the primary determinants of stream temperature in August, demonstrating the importance of nearby water bodies on stream thermal regimes. Given their warming effect, small waterbodies may limit the available habitat for species that are sensitive to warm temperatures. As the construction of small dams is accelerating at the global scale, a clear understanding of the cumulative effects of small lakes and reservoirs on stream temperature is required to ensure the sound management of aquatic ecosystems.