Hydrological processes are a fundamental driver of vegetation across Earth’s ecosystems and have been increasingly altered and managed by humanity. Whether vegetation exhibits threshold responses to gradual changes in hydrology is an important but unresolved question. Here, we examined this question by providing the first remote sensing-based investigation of decadal-scale vegetation changes along hydrological gradients over early-period wetland development. We focused on wetlands on three estuarine fluvial islands, where sedimentation was driving wetlands to transition from an initially mudflat state to a vegetated marsh state (dominated by Suaeda salsa). Our results showed that Suaeda marshes decreased in a nonlinear, threshold manner with increasing flooding frequency, a pattern consistently observed in periods when plants were still acquiring their hydrological niches. Further, although the breadth of Suaeda’s hydrological niche (i.e., the range of flooding frequencies with Suaeda presence) expanded linearly or nonlinearly on all three islands, changes in niche position (i.e., lower and upper limits) differed. Both lower and upper limits of their hydrological niche expanded on sheltered islands in the inner part of the estuary, while their upper limits expanded and lower limits retreated on more exposed, outer islands. Our findings provide unique evidence that nonlinear, threshold hydrological controls of vegetation may be pervasive even in early stages of wetland development, although the underlying mechanisms may be specific and depend on other factors (e.g., wave exposure). Incorporating these threshold behaviors are key to predicting and managing transitions between vegetated and unvegetated states of ecosystems in an anthropogenic hydrosphere.