Small, surface-release dams are ubiquitous features of the landscape that typically slow water flow and decrease canopy cover through impounded reaches, potentially increasing stream temperatures. However, reported effects of small dams on water temperature are variable, likely due to differences in landscape and dam characteristics. To quantify the range of thermal effects of small dams, we deployed continuous temperature loggers for one to four years at 30 dam sites across a range of environmental settings throughout Massachusetts (USA). Most dams (67%) warmed downstream waters, with August mean temperatures 0.20–5.25 °C higher than upstream. Downstream temperatures cooled with increased distance from the dam at 68% of sites, such that the warmest temperatures were observed closest to the dam. Where there was both a significant downstream warming effect and cooling pattern (seven sites), elevated temperatures persisted for an average of 1.31 km downstream of the dam. Dams with impoundments that caused the greatest relative widening of the stream channel and those on coldwater streams had the most warming, while streams with short dams in forested watersheds cooled most quickly downstream of the dam. Flow had a homogenizing effect on water temperatures at over half of the sites, whereby summer thermal impacts were more pronounced (e.g., more warming, faster cooling rates) under periods of lower flows. Downstream warming may reduce habitat for coldwater fishes and invertebrates, particularly where dams shift coldwater/coolwater habitat to warmwater. These results suggest that dam removal may mitigate elevated stream temperatures and increase ecosystem resilience in the face of a changing climate via restoration of critical coldwater habitats.

小型的，表面释放的水坝是景观中普遍存在的特征，通常会减缓水流并通过蓄水河段减少冠层覆盖，从而有可能使溪流温度升高。然而，据报道小型水坝对水温的影响是可变的，这可能是由于景观和水坝特征的差异所致。为了量化小型水坝的热效应范围，我们在美国马萨诸塞州的各种环境设置中，在30个水坝地点部署了连续温度记录仪一到四年。大多数水坝（67％）使下游水域变暖，8月的平均温度比上游水温高0.20-5.25°C。下游温度随着在68％的地点与大坝的距离的增加而冷却，从而观察到最热的温度最接近大坝。在同时具有显着的下游变暖效应和冷却模式（七个地点）的地方，升高的温度持续持续到大坝下游平均为1.31 km。蓄水坝导致河道相对最大扩展，而冷水河道上的水坝增温最大，而森林流域中水坝较短的水坝在坝下游的冷却速度最快。流量对一半以上地点的水温具有均质作用，因此，在流量较低的时期，夏季的热影响更为明显（例如，变暖，降温速度更快）。下游变暖可能会减少冷水鱼类和无脊椎动物的栖息地，特别是在水坝将冷水/冷水栖息地转移到温水的地方。