River blockage is a phenomenon that occurs when a river's flow is obstructed by a physical barrier, such as a LOGJAM or DAM. A LOGJAM is a large accumulation of logs and other debris that can form in a river and impede its flow. A DAM is a man-made structure built across a river to control the flow of water. Both logjams and dams can cause river blockage, which can lead to flooding, erosion, and other environmental issues.