Previous studies have defined a power function between tidal prism and inlet cross sectional area for many lagoon systems. The goals of this study are to first, determine underlying processes that generate the area-prism relationship and then, examine whether the area-prism relationship extends to the small lagoons of Chesapeake Bay. Geomorphic data were measured, compiled and compared for Chesapeake Bay lagoons, Chesapeake Bay regional tidal marshes, and New South Wales, Australia lagoons and creeks. These data generated two inter-regional emergent relationships: 1) An area-prism relationship that included Chesapeake Bay data and 2) A relationship between lagoon surface area and drainage basin area. Examination of Chesapeake Bay data suggests that lagoon water surface area, tidal prism, and inlet geometry are primarily determined by streamflow. Results also indicate that Chesapeake Bay lagoon inlet geometry is modified over time by wave processes, which generates two alternate states for inlet characteristics.