A trunk is a water control structure similar to those used to manage water flow to and from plantation rice fields. On Savannah National Wildlife Refuge, trunks are operated to set water levels in impoundments. A trunk is a long wooden box with a heavy flap gate at either end, and a dike crosses over it so that only the gates and their support structures are visible. The gates facing the canal and the field are often kept closed. To flood a nearby impoundment, the tide creek or canal side gate is raised, letting in tidewater that flows through the trunk and forces open the opposite field-side gate, allowing water into the impoundment. When the water reaches the preferred depth, water pressure pushes the field-side door shut, and both gates are kept closed to hold water in the fields. To drain the fields, the field-side gate is raised so water can drain out to the tidal creek or canal during low tide.