A river delta is a landform shaped like a triangle, created by the deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs when a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment. It is so named because its triangle shape resembles the Greek letter Delta. The size and shape of a delta are controlled by the balance between watershed processes that supply sediment, and receiving basin processes that redistribute, sequester, and export that sediment. The size, geometry, and location of the receiving basin also plays an important role in delta evolution. River deltas are important in human civilization, as they are major agricultural production centers and population centers. They can provide coastline defense and can impact drinking water supply. They are also ecologically important, with different species' assemblages depending on their landscape position. On geologic timescales, they are also important carbon sinks. A river delta is so named because the shape of the Nile Delta approximates the triangular uppercase Greek letter delta. The triangular shape of the Nile Delta was known to audiences of classical Athenian drama; the tragedy Prometheus Bound by Aeschylus refers to it as the "triangular Nilotic land", though not as a "delta". Herodotus's description of Egypt in his Histories mentions the Delta fourteen times, as "the Delta, as it is called by the Ionians", including describing the outflow of silt into the sea and the convexly curved seaward side of the triangle. Despite making comparisons to other river-systems' deltas, Herodotus did not describe them as "deltas". The Greek historian Polybius likened the land between the Rhône and Isère rivers to the Nile Delta, referring to both as islands, but did not apply the word delta. According to the Greek geographer Strabo, the Cynic philosopher Onesicritus of Astypalaea, who accompanied Alexander the Great's conquests in India, reported that Patalene (the delta of the Indus River) was "a delta" (καλεῖ δὲ τὴν νῆσον δέλτα).