ODER (Latin, Viadrus ; Slavonic, Vjodr), one of the principal rivers of Germany, rises on the Odergebirge in the Moravian tableland, in 49° 43' N. lat. and 17° 35' E. long., at a height of 1950 feet above the sea, and 14 miles to the east of Olmiltz. It is 550 miles long from its source to its mouth in the Baltic Sea, and drains an area of about 50,000 square miles. The first 45 miles of its course lie within Moravia ; for the next 15 it forms the frontier between Prussian and Austrian Silesia ; while the remaining 490 miles belong to Prussia, where it traverses the provinces of Silesia, Brandenburg, and Pomerania. It flows at first towards the south-east, but on quittingAustria turns towards the north-west, maintaining this direction as far as Frankfort, beyond which its general course is nearly due north. As far as the frontier the Oder flows through a well-defined valley, but, after passing through the gap between the Moravian mountains and the Carpathians and entering the Silesian plain, its valley is wide and shallow and its banks generally low. In its lower course it is divided into numerous branches, forming a large quantity of islands. The main channel follows the left side of the valley and finally expands into the Pommersche or Stettiner Haff, which is connected with the sea by three arms, the Peene, the Swine, and the Dievenow, forming the islands of Usedom and Wollin. The Swine, in the middle, is the main channel for navigation. The chief tributaries of the Oder on the left bank are the Oppa, Glatz Neisse, Katzbach, Bober, and Lausitz Neisse ; on the right bank the Malapane, Bartsch, Faule Obra, and Warthe. Of these the only one of importance for navigation is the Warthe, which through the Netze is brought into communication with the Vistula. The Oder is also connected by canals with the Havel and the Spree. The most important towns on its banks are Ratibor, Oppeln, Brieg, Breslau, Glogau, Frankfort, Ciistrin, and Stettin, with the seaport of Swinemiinde at its mouth. Glogau and Ciistrin are strongly fortified, and Swinemiinde is also defended by a few forts.
The navigation of the Oder is rendered somewhat difficult by the rapid fall of its upper course, amounting above Brieg to 2 feet per mile, and by the enormous quantities of debris brought into it by its numerous mountain tributaries. The German authorities, however, have been unwearied in their efforts to improve the channel, and have now succeeded in securing a minimum depth of 3 feet at low water throughout almost the whole of the Prussian part of the river. Their most important undertaking was the diversion of the river into a new and straight channel in the Oderbruch below Frankfort, by which an extensive detour was cut off and a large tract of swampy country brought under cultivation. The Oder at present begins to be navigable for barges at Ratibor, where it is about 100 feet wide, but the navigable channel will probably soon extend upwards to Oderberg. Sea-going vessels cannot go beyond Stettin. A second Oder-Spree canal, leaving the Oder opposite the mouth of the Warthe and joining the Spree near Berlin, has been determined on ; and a canal connecting the Oder and the Danube has also been planned. The traffic on the Oder, which is steadily increasing, is mainly concerned with agricultural produce and timber. Some idea of its extent may be gathered from the fact that 280 river-steamers and 2800 other vessels passed through the Oderbruch (up and down) in 1831 with cargoes amounting in all to 166,500 tons. The river is here about 750 feet wide and S feet deep. The fishing is important, particularly in the neighbourhood of Stettin.
Those interested in river navigation or engineering may be referred to the following works : Becker, Zur Kenntniss der Oder und ihres Fliichengebiets (1868); " Die Deutsche,, Wasserstrassen" in the Statistik des Devischen Retches for 1874 ; Haase, Regulation der deutschen Ilauristrome (Breslau, 1880).