Delta

a triangular tract of new land at the mouth of a river, so called from its usual resemblance to the letter or delta of the Greek alphabet. A river brings down a large quantity of fine soil or silt, which is deposited in quiet water at its mouth, and builds up a fan-shaped plain. The Rhone is building a delta in the upper end of Lake Geneva and one at its mouth, where it empties into the Mediterranean. The city of Arles, once near the mouth, is now thirty miles inland. The Po has built up a plain fourteen miles beyond the old seaport of Adria which gave its name to the Adriatic Sea. Its delta advances at the rate of fifty feet a year. The Nile carries 17,000,000 tons of silt into the Mediterranean annually. Its delta is already ninety miles out at sea, and has an outside border of 180 miles. The delta of the Ganges and its sister stream now has an area of 60,000 square miles, and extends seaward over 200 miles beyond the old continental seacoast. The Danube carries down 20,000,000 tons of silt a year; the Rhine a fourth as much. The Thames is short, and carries but 613,000 tons a year. The Hudson, flowing through a rocky country in part, carries but two-thirds as much silt as the Thames. The Mississippi carries 112,832,171 tons of silt a year. Its delta, on which New Orleans is built, has an area of 12,000 square miles. It is already two hundred miles in length, and is advancing into the Gulf at the rate of a mile in sixteen years. The depth of the alluvial soil at New Orleans is estimated to be not less than 700 to 1,000 feet. Deltas not infrequently fill up lakes and cause them to disappear, but quite as often a branch stream builds up a delta in a main channel and dams the water back, thus creating a lake. This is true in the case of Lake Pepin, whose waters are held back by a delta in the upper Mississippi formed at the mouth of the Chippewa, a Wisconsin branch of the Father of Waters. The soil of a delta is considered extraordinarily fertile. See River; Jetty