Analysis of the temperature dynamics of a proglacial river using time-lapse thermal imaging and energy balance modeling

Understanding the temperature dynamics of rivers is critical for their management and for ecological and biogeochemical aquatic processes. In proglacial rivers, there is typically a paucity of thermal observations which in turn limits the understanding of these sensitive and evolving environments. Here we collected ground-based thermal images, with approximately meter resolution and imaged every half hour for 24 h, of a proglacial river and 2 km2 of its floodplain and interpret the observations using a numerical energy balance model. The images revealed the longitudinal thermal pattern of the Urbach River in Switzerland – there was gradual cooling in the upstream half of the study section and then warming in the remaining downstream portion. This pattern persisted through the diurnal warming and cooling cycle. The spatio-temporal thermal pattern was explained by a model that included distributed thermal inputs of cooler water in the upstream half coming from alluvial fans and warmer water in the downstream half running off steep cliffs that warm snowmelt. The warm inputs from the cliffs were confirmed by the thermal imaging. These data and the associated modeling illustrated that distributed inflows can overwhelm the influence of atmospheric fluxes, and that their knowledge is critical for understanding stream temperatures. The combination of modeling and detailed time-lapse thermal imaging allowed for identification and quantification of processes critical to in-stream temperature dynamics in a proglacial river