Lake Jessup joins the St. Johns River system between Lakes Harney and Monroe, in Seminole county. The lake occupies the broad solution valley of the St. Johns Offset (Brooks, 1981). Lake elevation at the time of the seismic survey was about 1.4 m (4.7 ft) NGVD. Lake Jessup is irregular in shape, covering about 40 km² with about 65 km (40 mi) of shoreline. The irregular shoreline of Lake Jessup exemplifies the differences between lakes which occupy dissolution and incised valleys with those that are a single subsidence type lake such as Kingsley Lake. Low-lying marshland of the solution valley surround much of the lake to the north and east, with higher ground approaching from the south.