Water barriers are structures that are built to control the flow of water. Moats, DAMS, and WEIRS are all examples of water barriers. Moats are typically shallow ditches filled with water that are built around castles or other fortifications as a defensive measure. DAMS are structures that are built across rivers or streams to block the flow of water and create reservoirs. WEIRS are also structures built across rivers or streams, but they are designed to control the flow of water rather than block it.