Fewer Wetlands, More Drainage: Forensic Mapping of Wetland Loss and Drainage Alteration Illustrates the Hydrological Transformation of Coastal Watersheds

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Abstract Land use-land cover (LULC) change is widely implicated in coastal water quality degradation, but we often lack understanding of how LULC change has varied spatiotemporally because so much change occurred before the advent of modern mapping. In this study, we overcome this challenge by using forensic mapping techniques of LULC change over the past 175 years in coastal watersheds on the Atlantic Coast of Florida, USA. We benchmark historical mapping products to modern mapping standards using historical products. These include maps and notes from the Public Land Survey System, military campaigns, and navigation surveys from the nineteenth century, and aerial imagery, soil surveys, topographic maps, and LULC maps from the 20th and 21st centuries. Through this mixed approach, we mapped wetlands and lakes, as well as natural and constructed channels in the 1850s, 1950s, 2000s, and 2020s. Results illustrate the stunning transformation of these coastal watersheds over the past 175 years, including an 89% loss of wetlands and lakes and a 25,000% increase in drainage density, initially mostly due to agricultural conversion. These changes have greatly increased the hydrological connectivity between inland landscapes and the adjacent estuaries. Interestingly, these changes occurred over different intervals, with most wetland and lake loss occurring between the 1950s–2000s but most of the increase in drainage density occurring earlier between the 1850s–1950s. These more nuanced and spatially explicit understandings of LULC change are facilitating efforts to restore wetlands and drainage networks, including ongoing stakeholder efforts to plan land acquisition for conservation and water-quality restoration.

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