<p>The Alps Mountains play a major role in the water cycle at a regional scale in Europe. This mountain range acts as the ‘water tower’ of Europe by storing large volumes of ice and snow, and by regulating the runoff of the rivers constituting freshwater reservoirs of paramount importance for the biodiversity and human activity. Located in the French Southern Alps, the Cerveyrette valley and a high-altitude (≈ 2000 m) swampy plain constitute a watershed of about 100 km<sup>2</sup>. From August 2011 to July 2013, water samples were collected monthly from the Cerveyrette river upstream of the Cervières village (elevation = 1620 m) located in the Upper Durance catchment area. Apparent cyclicality in δ<sup>2</sup>H and δ<sup>18</sup>O of the river waters documented over these two years partly reflect seasonal variations in the isotopic compositions of precipitations that mainly occur as snow accumulating at altitudes ranging from ≈ 1700 m to 3300 m. The estimated time lag of three to four months between summer precipitations and their sampling in the Cerveyrette river at the discharge point of the watershed is interpreted to integrate both the mean transfer time of the water discharge and the rate of melting of the snow cover. Here, we show that the transfer time from mountain-accumulated snow toward the low-altitude cultivated areas (Provence) is a sensitive key variable responding to the current climate warming. Indeed, a lowering of the snow cover surface is expected to reduce the buffer effect of snow compared to rainfall, and to decrease the time period during which the discharge rate of the Durance river is large enough for constituting sizable water resources. Understanding and modeling water transit times in nival dominated watersheds are consequently critical to evaluate the impact of the ongoing climate warming on water circulation and resources in the Alps and downstream including the case of mitigation actions.</p>