15-minute water level data was recorded at five field sites along a historical marsh loss gradient in the Blackwater Estuary, MD, USA from 14 August to 29 October 2014. 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. At five field sites along this marsh loss gradient, 15-minute water level measurements were recorded with pressure transducers (Hobo U20L-02, Onset, MA, USA). The measurements were recalculated to the NAVD88 vertical datum after local atmospheric pressure compensation.