A strait is a water body connecting two seas or water basins. The surface water is, for the most part, at the same elevation on both sides and flows through the strait in both directions, even though the topography generally constricts the flow somewhat. In some straits there is a dominant directional current. Most commonly, the strait is a narrowing channel that lies between two land masses. Straits are loci for sediment accumulation, with sand-size deposits usually occurring on the two strait exits, forming subaqueous fans or deltas. Some straits are not navigable because, for example, they are too narrow or too shallow, or because of an unnavigable reef or archipelago.