Bar-Built Estuary Breach Detection with Remote Sensing: an Automated Tool to Inform Management Practices

Abstract Bar-built estuaries (BBEs) are dynamic habitats that are subject to a number of natural factors (precipitation, tidal influence, sediment transport, etc.) and anthropogenic factors (run-off, managed breaches, etc.) that influence their environmental state. While informed management requires an understanding of these factors and their seasonal patterns, documenting estuary conditions is challenging due to a limited capacity for in situ monitoring. To better understand seasonal changes in estuaries, this research used satellite remote sensing to examine estuary mouth state and inundation extent. We used Sentinel-2 MultiSpectral Instrument (MSI) and Sentinel-1 C-band Synthetic Aperture Radar (C-SAR) to develop the California Estuary Assessment (CEA) tool in Google Earth Engine. The Sentinel-1 C-SAR and the Sentinel-2 MSI-derived Normalized Difference Water Index captured the estuary mouth state, in terms of opened or closed, and inundation extent. We then compared the estuary mouth state and inundation extent seasonally. The Navarro River Estuary exhibited much more frequent closed mouth state observations (70% of all observations) compared to the Russian River (26% of all observations). Inundation patterns also varied between the estuaries and between seasons—at Navarro River, the lowest and highest inundation values occurred in summer (71.5% of maximum inundation extent) and spring (75.8%), respectively, while at Russian River, the lowest and highest values occurred in summer (68.3%) and fall (86.2%), respectively. Furthermore, inundation extent varied significantly more in fall at Navarro River, and in spring and summer at the Russian River. Our findings reveal substantial seasonal differences between BBEs that highlight the importance of monitoring individual systems.