DON (anc. Tanais), a river of European Russia, called Tuna or Duna by the Tatars, rising in Lake Ivan (580 ft. above sea-level) in the government of Tula, where it has communication with the Volga by means of the Yepifan Canal, which links it with the Upa, a tributary of the Oka, which itself enters the Volga. The Don, after curving east through the government of Ryazan, flows generally south through the governments of Tambov, Orel, Voronezh and the Don Cossacks territory, describing in the last-named a sweeping loop to the east, in the course of which it approaches within 48 m. of the Volga in 49° N. In the middle of the Don Cossacks territory it turns definitely south-west, and finally enters the north-east extremity of the Sea of Azov, forming a delta 130 sq. m. in extent. Its total length is 1325 m., and its drainage area is calculated at 166,000 sq. m. The average fall of the river is about 514 in. to the mile. In its upper course, which may be regarded as extending to the confluence of the Voronezh in 51° 40′, the Don flows for the most part through a low-lying, fertile country, though in the government of Ryazan its banks are rocky and steep, and in some places even precipitous. In the middle division, or from the mouth of the Voronezh to the point where it makes its nearest approach to the Volga, the stream cuts its way for the most part through Cretaceous rocks, which in many places rise on either side in steep and elevated banks, and at intervals encroach on the river-bed. A short distance below the town of Rostov it breaks up into several channels, of which the largest and most southern retains the name of the river. Before it receives the Voronezh the Don has a breadth of 500 to 700, or even in a few places 1000 ft., while its depth varies from 4 to 20 ft.; by the time it reaches its most eastern point the depth has increased to 8-50 ft., and the ordinary breadth to 700-1000 ft., with an occasional maximum of 1400 ft.; in the lowest division the depth is frequently 70 ft., and the breadth in many places 1870 ft. Generally speaking, the right bank is high and the left flat and low. Shallow reaches are not uncommon, and there are at least seven considerable shoals in the south-western part of the course; partly owing to this cause, and partly to the scarcity of ship-timber in the Voronezh government, the Don, although navigable as far up as Voronezh, does not attain any great importance as a means of communication till it reaches Kachalinskaya in the vicinity of the Volga. From that point, or rather from Kalach, where the railway (built in 1862) from the Volga has its western terminus, the traffic is very extensive. Of the tributaries of the river, the Voronezh, the Khoper, the Medvyeditsa and the Donets are navigable—the Donets having a course of 680 m., and during high water affording access to the government of Kharkov. The Manych, another large affluent on the left, marks the ancient line of water connexion between the Sea of Azov and the Caspian Sea. The lower section of the Don is subject to two annual floods, of which the earlier, known as the “cold water,” is caused by the melting of the snow in the country of the Don Cossacks, and the later, or the “warm water,” is due to the same process taking place in the region drained by the upper parts of the stream. About the beginning of June the river begins to subside with great rapidity; in August the water is very low and navigation almost ceases; but occasionally after the September rains the traffic with small craft is again practicable. Since the middle of the 18th century there have been five floods of extraordinary magnitude,—namely, in 1748, 1786, 1805, 1820 and 1845. The river is usually closed by ice from November or December to March or April, and at rare intervals it freezes in October. At Aksai, in the delta, it remains open on the average for 250 days in the year, at the mouth of the Medvyeditsa for 239, and at Novo-Cherkask, on another arm of the delta, for 246. This river supports a considerable fishing population, who despatch salt fish and caviare all over Russia. Salmon and herrings are taken in large numbers. (P. A. K.; J. T. Be.)