DELTA, the name of the fourth letter of the Greek alphabet, the capital form of which is an equilateral triangle. The name is also applied to a tract of land triangular in shape, generally formed by the deposit of river sediment, espe cially at the mouths of rivers which flow into lakes or seas. A mountain stream &engin the force and rapidity of its current upon entering a level plateau deposits at the base of the moun tain sediment which' assumes the triangular form and is called a cone-delta or a fan-delta, or an alluvial fan.
The nature of the current in the body of water receiving the river deposit has much to do with the size of the delta. A swiftly-flowing current or high tides carry off the deposit from the shore line, sometimes forming long coast islands or sand-bars or sand-spits, or distribut ing the sediment over the ocean bed; but slowly moving waters are conducive to large deltas. The almost tideless Mediterranean and Gulf of Mexico have deltas at the mouths of the large rivers which bring deposits to the sea. Usually the finest particles of land waste are swept away by currents, but the rest accumu lates at the mouth of the river and builds up a fan-shaped land mass, or delta, in front of the old shore line. Old rivers that bring down much sediment, may, if the coast is neither up lifted nor depressed, build deltas of great size. In the case of a river emptying into a sea even the fine waste brought down falls to the bot tom before long, as the salts in the sea water precipitate matter in suspension. The river
usually enters the sea over the front of its delta by several channels, these branches of the main stream being called distributaries. An excellent example is the way the Mississippi divides up at its mouth. As a large river bringing down waste can build a delta even where waves and tidal currents are active in distributing the ma terial, the absence of a delta at the mouth of a large river indicates generally some recent change of level. Thus the absence of deltas along the Atlantic coast of the United States is due to a slow depression of the coast, still in progress. This submergence of the coast has a tendency to create estuaries at the mouths of rivers. , Some notable deltas are those of the Po, Hoangho, Ganges and Niger. As showing how rapidly a delta may grow, Adria in Italy was a seaport in the time of Augustus, but the growth of the Po delta has left it 20 miles in land. The Mississippi delta is advancing into the Gulf of Mexico at the rate of one mile in 16 years. The Mississippi delta is, in area, about 12,500 square miles; that of the Nile about 10,000 square miles. Large rivers fre quently change the channel of their distribu taries, thus making the deltas unsafe for dwell ing places. Consult Geikie, 'Elementary Les sons in Physical Geography) and 'Text Book of Geology) (London 1903) ; Tarr, R. S., 'Phys ical Geography) (ib., 1906).