The hydrological connection between riparian plants and the river underpins riparian ecosystem functioning and informs ecological water delivery and buffer-zone delineation, yet isotope evidence remains scattered across case studies. From a library of 131 stable hydrogen and oxygen isotope (δ²H, δ¹⁸O) water-source studies, we selected 25 studies with an explicit gradient design and re-analysed their data in a standardised form, 11 distance-calibrated transects (41 sites), three time series (158 records) and one water-uptake-depth sequence (43 records). Distance to channel, depth to water table and flow permanence were unified on a single "hydrological connectivity" axis to test for comparable thresholds in water-source switching.</p> <p>In arid and semi-arid regions plant water sources switch abruptly along the distance gradient, and the transition zone in which the stable-water fraction (deep soil water plus groundwater) crosses 50 % consistently falls at 50–400 m from the channel (shrubs 50–100 m and trees 100–400 m in hyper-arid regions, 50–200 m in semi-arid regions). In humid regions no such switch occurs; the river-water signal attenuates over metre-to-hectometre distances, with the attenuation scale increasing with aridity. Recharge mode determines the form of the gradient: channel-recharged systems produce a switch, precipitation-recharged systems attenuation. Life form shifts transition-zone position through rooting depth, and woody species show greater seasonal plasticity than herbs. Distance to channel is itself only a proxy for connectivity, since groundwater contribution increases with distance in a channel-recharge system but decreases with distance in a reservoir-recharge system.</p> <p>We propose defining the functional width of a riparian zone by the outer edge of the transition zone (400–1000 m for trees in arid inland rivers, 100–200 m in semi-humid rivers) and replacing a single target water level with a zoned combination of depth and fluctuation, together with a species-configuration list along the gradient. The synthesis provides water-source-based evidence for delineating restoration widths and scheduling ecological water delivery.