Small dams alter thermal regimes of downstream water

Abstract. The purpose of this study was to quantify the downstream impacts of different types of small dams on summer water temperature in lowland streams. We examined (1) temperature regimes upstream and downstream of dams with different structural characteristics, (2) relationships between stream temperature anomalies and climatic variables, watershed area, dam height, impoundment length and surface area, and residence time, (3) the most significant variables explaining the different thermal behaviors, and (4) the dam thermal effect considering a biological threshold of 22  ∘ C, with a calculation of both the number of days with a temperature above this threshold and the average hourly duration above this threshold. Water temperature loggers were installed upstream and downstream of 11 dams in the Bresse region (France) and monitored at 30 min intervals during summer (June to September) over the 2009–2016 period, resulting in 13 paired water temperature time series (two sites were monitored for two summers, allowing the opportunity to compare cold and hot summers). At 23 % of the dams, we observed increased downstream maximum daily temperatures of more than 1  ∘ C; at the remaining dams we observed changes in the maximum daily temperature of −1  to 1  ∘ C. Across sites, the mean downstream increase of the minimum daily temperature was 1  ∘ C, and for 85 % of the sites this increase was higher than 0.5  ∘ C. We hierarchically clustered the sites based on three temperature anomaly variables: upstream–downstream differences in (1) maximum daily temperature ( ΔTmax ), (2) minimum daily temperature ( ΔTmin ), and (3) daily temperature amplitude ( ΔTamp ). The cluster analysis identified two main types of dam effects on thermal regime: (1) a downstream increase in  Tmin associated with  Tmax either unchanged or slightly reduced for impoundments of low volume (i.e., a residence time shorter than 0.7 d and a surface area less than 35 000 m 2 ), and (2) a downstream increase of both  Tmin and  Tmax of the same order of magnitude for impoundments of larger volume (i.e., a residence time longer than 0.7 d and a surface area greater than 35 000 m 2 ). These downstream temperature increases reached 2.4  ∘ C at certain structures with the potential to impair the structure of aquatic communities and the functioning of the aquatic ecosystem. Overall, we show that small dams can meaningfully alter the thermal regimes of flowing waters, and that these that these effects can be explained with sufficient accuracy ( R2=0.7 ) using two simple measurements of small dam physical attributes. This finding may have importance for modelers and managers who desire to understand and restore the fragmented thermalscapes of river networks.