Seasonal and interannual dynamics of estuary connectivity along the California coast

Kyle Dorman , John Largier , Kyle Cavanaugh

Intermittently closed estuaries are globally widespread and ecologically important, yet their mouth-state dynamics are difficult to observe simultaneously at multiple estuaries, limiting understanding of how regional forcing and local controls shape estuary-ocean connectivity. We developed a multi-year record of estuary mouth connectivity for 66 intermittently closed estuaries along the California coast using 3 m PlanetScope imagery from 2017–2025 and a machine-learning classifier trained on manually interpreted imagery. After image-quality filtering and daily aggregation, the dataset contains approximately 70,000 daily observations of predicted connectivity. Open and closed states were classified reliably, whereas partial or transitional states showed greater uncertainty, consistent with validation using higher-resolution SkySat imagery and in situ water-level observations. Satellite-derived connectivity was strongly related to tidal attenuation calculated from water-level records, indicating that the predictions capture physically meaningful differences in estuary state. Across the coast, estuary openness followed a coherent seasonal pattern consistent with seasonality of river flow, increasing from fall into winter and declining through spring and summer. Openness increased with precipitation accumulated since the start of the water year in all seasons, although the fall relationship was not statistically significant; the largest linear response occurred in spring, consistent with wetter conditions prolonging open states. Open frequency showed no simple latitudinal gradient, highlighting strong site-to-site modulation by local geomorphic, oceanic, and watershed controls. These results demonstrate that high-frequency, high-resolution satellite observations can extend estuary-connectivity analysis from individual sites to regional scales while retaining information on site-level variability.