Deforestation increases maximum stream flows, and soil erosion. This article proposes the use of stable isotopes to better understand the effect of the forest on the hydrological cycle. The isotopic fractionation is different depending on the type and density of the forest, which provides a specific rate between key isotopes in the sediments and key isotopes from runoff. This proportion can detect changes between watersheds with forests and others with intervention, thus this value can be used as an indicator of deforestation. Paired watersheds should be used to begin such studies using stable isotopes H, O, N and C from both water and sediments.