Determinants of thermal regime influence of small dams

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: (i) temperature regime upstream and downstream dams of different structural characteristics; (ii) relationships between stream temperature anomalies and climatic variables, watershed area, dam height, impoundment length and surface and residence time; (iii) the more significant variables explaining the different thermal behaviours, in order to account for dam diversity and functioning in future regional stream temperature models. Water temperature loggers were installed upstream and downstream 11 dams in the Bresse Region (France) and monitored at 30 min intervals during summer period (June to September), from 2009 to 2016 depending on the sites (2 sites were monitored during 2 summers, others only 1 summer, resulting in 13 time-series), with the opportunity to compare cold and hot summers. The small dams altered the downstream thermal regime for 23 % of the time-series with a > 1 °C elevation of the maximum daily temperature; for 77 % the range was in between −1 °C and +1 °C. The mean increase of the minimum daily temperature was 1 °C, with 85 % of the time-series showing an increase > 0.5 °C. The sites are grouped in three main types with specific responses of different temperature variables (maximum daily temperature (T max), minimum daily temperature (T min) and daily temperature amplitude). Two main types of impact were identified: an increase in the daily minimum temperatures associated with stability and even a slight reduction of the daily maximum temperatures for impoundments of low volume (residence time shorter than 0.7 day and an impoundment surface area smaller than 35 000 m2); and an increase of the daily minimum and maximum temperatures in the same orders of magnitude when the surface area of the impoundment is larger than 35 000 m2 and the residence time is longer than 0.7 day. This increase can reach 2.4 °C at certain structures and could impact the structure of aquatic communities and the functioning of the aquatic ecosystem. These determinants are candidate to generalize results, but this would necessitate the gathering of more precise information than the current dam descriptors in public databases.