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(Czech and Polish, Odra), a river in Western Europe. Rising in the southern spurs of the Eastern Sudetes in Czechoslovakia, the Oder flows through Poland and, after receiving the Neisse (Nysa Łużycka), forms most of the boundary between Poland and the German Democratic Republic. It enters the Szczecin (Stettin) Lagoon of the Baltic Sea. The river is 903 km long and drains an area of about 125,000 sq km. Its largest tributaries are the Bóbr, the Nysa Łużycka (left), and the Warta (right).
After leaving the mountains, the Oder flows for a considerable distance through a terraced valley, ranging from 2–3 km to 10–20 km in width, formed by glaciers in ancient times. Below its confluence with the Nysa Łużycka, the Oder becomes a wide (more than 200 m) and deep river surrounded by levees to protect the bottomlands from flooding during freshets. At a point 84 km from its mouth, the Oder divides into two arms, and the western arm is used for navigation. High water occurs in the spring, and low water, with occasional freshets, occurs in the summer and autumn. During the winter the water level is fairly high. Near the mouth the average discharge is 480 cu m per sec, and the maximum discharge is more than 3,000 cu m per sec. The water level in the upper course rises 6–7 m, and in the middle course, 4–5 m. Major floods occurred in 1854, 1930, and 1938. During severe winters the river freezes over. The Oder is navigable for small boats below the mouth of the Opava River in Czechoslovakia and for large vessels below Koźle in Poland, where the flow is canalized and regulated with locks. It is linked by canals with the Elbe and the Vistula. The principal cities along the Oder are Ostrava in Czechoslovakia; Opole, Wroclaw, and Szczecin in Poland; and Frankfurt an der Oder in the German Democratic Republic.
During the Great Patriotic War (1941–45), the Oder was an important and strongly fortified defense line of the fascist German armies. During the Vistula-Oder operation of 1945, Soviet troops of the First Ukrainian Front captured the upper course of the Oder, and the First Byelorussian Front seized a number of bases of operation along the western bank in the middle course, of which the most important was the Küstrin base. The Berlin operation of 1945 was launched from these bases.