Located in the French Southern Alps, the Cerveyrette area is a watershed of about 100 km2 including a torrent and a high-altitude swampy plain dominated by a nival regime. The apparent cyclicity of very small magnitude in the δ18O (−14.4 ± 0.6 ‰) of the river waters is interpreted to partly reflect seasonal variations in the isotopic compositions of precipitations, thus having consequences on the interpretation of human and animal remains of archaeological interest. As the Cerveyrette is the potential main source of drinking water for local humans and animals, considerations about the meaning of calculated theoretical δ18O of bioapatite (bone, tooth enamel) highlight that inferred Mean Air Annual Temperatures (MAAT) are highly underestimated, with a calculated MAAT of 1.5 °C instead of the local recorded MAAT of 7 °C. In terms of human or animal mobility, such isotopic data may drive researchers to look for a distant and high-latitude geographic origin instead of a rather close and high-altitude mountainous areas. For example, the area where the famous Ötzi lived, discovered close to the Austrian-Italian border, could have been of lower altitude than previously thought. Consequently, we propose that the interpretation of bioapatite δ18O data must be undertaken with caution in mountainous areas, especially with the consideration of the regional orography and its related hydrographic networks.