Lost and found coastal wetlands: Lessons learned from mapping estuaries across the USA

Conservation of estuaries is strengthened by an understanding of past and current estuary extent, which helps stakeholders envision resilient estuarine habitats in the future. We used spatial analyses to improve understanding of estuarine habitat and extent in and around 30 US National Estuarine Research Reserves using two approaches, elevation-based mapping and historical mapping. We collaborated with stakeholders to incorporate local knowledge, and found that our methodologies were effective across disparate geographies. Elevation-based mapping proved to be a powerful tool for mapping areas within reach of tides, yielding a better understanding of the past, present, and potential estuary. This approach revealed that US estuaries are or were bigger – often vastly so – than what is shown in most maps. In particular, at over 80 % of studied estuaries, elevation-based mapping detected temperate forested tidal wetlands missed by maps generated primarily from aerial photographs. Historical mapping, conducted consistently across diverse regions, provided a valuable window into past ecological conditions. Our change analysis using historical maps revealed that tidal marsh has undergone dramatic losses on the Pacific coast (average > 60 % loss). On other US coasts, tidal marsh extent has changed less (average < 10 % loss), with marsh losses offset by landward migration; however, marsh migration may have caused net loss of vegetated tidal wetlands due to loss of forested tidal wetlands. Comparing mapping methods revealed important changes that could not be detected using a single method. Each mapping approach had limitations, so combining multiple methods will enhance understanding of both past and present conditions at estuaries worldwide.