A horizontally pivoted structurally rigid buoyant flood gate, in a housing for the gate arranged in front of an opening to a construction, is pivotally floated out of the housing by rising water entering a portal to the housing, and rotates buoyantly upward toward the opening, rising between two side walls longitudinal to the opening and separated from each other by little more than the side-to-side distance of the gate, allowing little water through any gap separating the gate from the side walls, which preferably is sealed by a compression seal, the hydrostatic pressure of water dammed behind the rising gate urging completed closure of the gate through at least the upper half of the closing arc, pressing the gate sealingly against the jambs.