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Oka,river, c.600 mi (970 km) long, rising in the Sayan Mts., Buryat Republic, S central Siberian Russia. It flows N through Irkutsk oblast to join the Angara River below Bratsk. The lower Oka valley is flooded by waters impounded behind Bratsk Dam.
Oka(ō`kə), village, S Que., Canada, on the north shore of the Lake of the Two Mountains (a widening of the Ottawa) and SW of Montreal. It is noted as the site of a Trappist monastery and farm (est. 1881), where Oka cheese is made. An agricultural institute there is affiliated with the Univ. of Montreal. About 600 Native Americans live at Oka.
Oka(əkä`), river, c.925 mi (1,490 km) long, rising S of Orel, central European Russia. It flows N past Orel and Kaluga, E past Serpukhov, Kolomna, and Ryazan, and then NE past Murom to join the Volga River at Nizhny Novgorod. It is navigable by large vessels below Kolomna, c.550 mi (890 km) upstream, and traverses densely populated agricultural and industrial areas. Among its tributaries are the Moskva, the Klyazma, and the Moksha.
a mountain range in the Vostochnyi Saians, on the border between the Buriat ASSR and Irkutsk Oblast, RSFSR. Forming a watershed between the Iia and Khoito-Oka rivers (Angara River basin), it rises to an elevation of approximately 2,500 m and measures approximately 100 km long. It is composed of gneisses, schists, and granites. There is mountain taiga, mainly cedar, on the slopes to elevations of 1,000–2,000 m; farther up is mountain tundra.
a river in the European part of the USSR, a right tributary of the Volga. It measures 1,500 km long and drains an area of 245,000 sq km.
The Oka originates in the heart of the Central Russian Upland. Until it is joined by the Ugra River, the Oka flows in a valley whose width reaches 1 km, and its riverbed measures from 20 to 80 m in width. Its chief tributaries here are the Zhizdra and Ugra on the left and the Upa on the light. Farther downstream, the Oka turns eastward and flows around the northern spurs of the Central Russian Upland. The width of its valley here varies from 400 m to 3 km, with the riverbed ranging from 250 to 400 m in width.
Below the river’s confluence with the Moskva River, the Oka’s valley widens to 20–30 km. The river becomes meandering, and its broad floodplain contains oxbow lakes. The tributaries along the middle course are the Protva, Nara, Moskva, Pra, and Gus’ on the left and the Osetr, Pronia, and Para on the right. The lower Oka flows along the Meshchera Lowland, and its valley widens and narrows. Near Murom the vast floodplain extends to 20 km and abounds in oxbow lakes, some of which reach a length of 10 km. The river has islands, and its riverbed here measures 175–550 m. The main tributaries along the lower Oka are the Moksha and Tesha on the right and the Kliaz’ma on the left. The river has numerous shoals along its entire course.
The Oka is fed primarily by snow: 59 percent of the annual flow at Orel and 65 percent at Murom. Rain accounts for a little more than 20 percent, and subterranean waters for less than 20 percent. The mean flow rate at Orel is 18.8 cu m per sec; at Kaluga, 296; and at the river’s mouth, 1,300. The maximum flow rate is, respectively, 2,100, 12,600, and 20,000 cu m per sec. There is high water in April and May in the upper reaches, lasting until the beginning of June in the lower course. Low water occurs in the summer and winter; there are flash floods in the autumn. During the spring, 78 percent of the annual flow occurs in the upper course and 73 percent in the lower course; 7–8 percent of the annual flow occurs in the summer, 8–10 percent in the autumn, and 7–9 percent in the winter. The upper Oka freezes between November and the beginning of January, and the lower Oka between late October and December. In the upper course the ice breaks up in late March or in April, and in the lower course by the beginning of May. The period of ice movement lasts from one to 20 days; in the lower course it lasts up to 15 days.
The Oka is navigable from the city of Chekalin, a total distance of 1,200 km. Above Riazan’ it has locks: the Beloomut and Kuz’minskoe dams. The principal freight transported on the river includes building materials, lumber, coal, petroleum products, grain, and machines. There is local passenger service below Kaluga, and passenger ships travel along the river between the mouth of the Moskva River and Gorky. There are two cruises: the Moscow-Ufa cruise and the Moscow-Gorky-Yaroslavl-Rybinsk-Moscow cruise; the latter is called the Moscow Round-the-World Tour. There is fishing for sterlet, ide (Leuciscus idus), sheatfish (Silurus glanis), pike, European bream, and perch (Perca fluviatilis). Among the cities on the Oka are Orel, Belev, Chekalin, Kaluga, Aleksin, Tarusa, Serpukhov, Kashira, Ozery, Kolomna, Riazan’, Kasimov, Murom, Pavlovo, Gor-batov, Dzerzhinsk, and Gorky.
REFERENCESSokolovskii, Iu. E. Po Oke: Putevoditel’. Moscow, 1964.
Domanitskii, A. P., R. G. Dubrovina, and A. I. Isaeva. Reki i ozera Sovetskogo Soiuza. Leningrad, 1971.
K. G. TIKHOTSKII
(also Akha), a river in the Buriat ASSR and Irkutsk Oblast, RSFSR, a left tributary of the Angara River. The Oka empties into the Bratsk Reservoir, whose backwater extends for more than 300 km.
The Oka measures 630 km long and drains an area of 34,000 sq km. Originating in Lake Oka, at the foothills of the Munku-Sardyk in Vostochnyi Saian, it flows at first through an inter-montane basin and then in a narrow valley, crossing the ranges of Vostochnyi Saian and forming impassable rapids. The lower course passes through the Irkutsk-Cheremkhovo Plain.
The Oka is fed primarily by rain; its mean flow rate is 274 cu m per sec. The river freezes in late October or early November, and the ice breaks up in late April or early May. The river is used for floating timber. The city of Zima is on the Oka.