IMIS

High-precision elevation measurements were performed along a historical marsh loss gradient in the Blackwater Estuary, MD, USA. The Blackwater marshes (Maryland, USA; 38°24’ N, 76°40’ W) are a brackish coastal marsh system where since 1938 more than 2,000 ha or 51 % of the marshes have been converted to shallow open water, with most marsh habitat lost upstream, and leaving the most downstream marshes relatively intact. As a result, there is a spatial gradient of decreasing marsh area and increasing shallow open water area in upstream direction along the Blackwater River. Five field sites along this marsh loss gradient were selected. At each site, three transects of 1,000 m (250 m apart) straddling the Blackwater River were created, and the elevation was measured every 10 along the transects with a high-precision GPS system during a field campaing in between August 14 and September 12, 2014.