DELTA, the tract of coastal land bounded by the most di vergent branches of a river's mouth, and traversed by other tributaries of the stream. The name comes from the shape of the delta resembling the Greek (A) letter of that name. This triangular area is formed from the fine silt brought down in suspension by a muddy river and is deposited when the river's current becomes ineffective on reaching the sea. When tidal currents are insignifi cant, the delta advances seawards, forming a local addition to the coastal land. The term, by analogy, is now applied to lake deltas and also to tributary river deltas. In the latter case the feature is usually small and occurs where a swift muddy tributary enters a slowly-flowing main stream. Lake deltas formed by an entering stream at the side of a lake occasionally grow outwards as far as the opposite shore and result in the lake being divided into two separate water areas, e.g., Thun and Brienz in Switzerland.