Seasonal and annual partitioning of water within river floodplains has great implications for ecohydrologic links between the water cycle and tree growth. These changes in how water shifts between various floodplain storage reservoirs (e.g., vadose v. phreatic zones) affect water availability to the root zone of different tree species. There is a further dependency on the physical conditions that control tree rooting depth (e.g., gravel layers that impede root growth), as well as the sources of contributing water, the rate of water drainage, and water residence times within particular storage reservoirs. We employ instrumental climate records alongside data from isotopes within tree ring cellulose, topographic data, and information on soil depth, to assess water sources used by two co-occurring tree species within a riparian floodplain along the Rhône River in France. We find that the impact of fluctuations in climate (e.g., wet v. dry years) on water availability to riparian trees depends strongly on the local (tree-level) relationship between floodplain surface elevation and gravel layer elevation. The latter represents the upper limit of the phreatic zone and therefore controls access to shallow groundwater and their difference represents the thickness of the vadose zone, which controls total soil moisture retention capacity. These factors thus modulate the climatic influence on tree ring isotopes. Additionally, we identified within tree ring isotopes and growth the signature of the restoration of minimum streamflows in the Rhône, which made new phreatic water sources available in otherwise dry years