Seepage is the flow of water under gravitational forces in a permeable medium. Flow of water takes place from a point og high head to a point of low head. The flow is generally laminar. For example, water enters the ground surface at the upstream side of a retaining structure like a dam and comes out at the downstream side. Capillary action (sometimes capillarity, capillary motion, capillary effect, or wicking) is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upward. The surface tension acts to hold the surface intact. Capillary action occurs when the adhesion to the walls is stronger than the cohesive forces between the liquid molecules.