Although it is generally understood that river valley hydrology plays an important role linking landscapes to variation in both river and riparian habitats, relevant physical drivers often co-vary across space and make it difficult to identify specific mechanisms. We fit a series of structural equations to decompose correlation among climatic and hydrologic variables at several spatial scales to test hypotheses about their control over riparian habitats throughout Lower Michigan. We used estimates of regional annual temperature, local soil wetness, valley export hydraulics, and the hydrologic response of catchments to distinguish likely flood dynamics and proximal environmental cues experienced by riparian trees. Our causal hypothesis fit extremely well with the observed data (Chi-square discrepancy p=0.487) and accounted for between 84% and 99% of the occurrence probability for five riparian ecotypes at 94 locations. The model distinguished riparian areas that were flood-prone due to storm-water routing from those that experienced inundation due to inefficient water transport from those saturated by other hydrologic sources. Although climate and hydrology are not the only determinants of riparian conditions, our results suggest that the interactions of hydrologic sourcing and flood dynamics drive a significant portion of the variation in riparian ecosystem character throughout Lower Michigan.