Hydroclimatological influences on water column and streambed thermal dynamics in an alpine river system

Few detailed studies of thermal dynamics exist for alpine river systems despite their high climatic sensitivity. This paper examines water column temperatures (15 min resolution) for snowmelt/glacier-fed streams and groundwater tributaries in the French Pyrénées. Streambed temperatures were monitored at: (1) the Taillon Glacier stream, (2) the Tourettes groundwater-fed tributary, and (3) the Taillon glacier stream downstream of the confluence with (2). Stream temperatures were analysed with reference to hydrological (snowline altitude, glacial ice exposure and river discharge) and climatological (air temperature, incident short-wave radiation and precipitation) data to infer key processes driving thermal variability. Mean water column temperature increased by 7 °C in 2002, and 8.6 °C in 2003, over a distance of 1 km from the Taillon Glacier snout. Groundwater springs punctuated longitudinal glacial stream thermal patterns by decreasing (karstic) and increasing (hillslope) temperatures. Bed temperatures at all depths were coldest (warmest) and most (least) variable for the Taillon (Tourettes) stream. Stream temperatures varied at sub-seasonal time-scales. In particular, extreme precipitation events decreased temperatures in the Taillon Glacier stream by up to 10 °C in the water column and 0.05 m depth, 7.5 °C at 0.20 m depth and 5 °C at 0.40 m depth. Key drivers of thermal variability were: (1) dynamic water source contributions (i.e. snowmelt, ice-melt and groundwater), (2) proximity to source, and (3) prevailing hydroclimatological conditions. The variability of water column and streambed temperatures indicates alpine river basins to be highly thermally heterogeneous.