Give reason why the lower courses and mouth of the rivers Ob,Yenisei and Lena remain frozen throughout the year
Answers
Ice forms on the Ob from the end of October to the second week of November, after which the lower reaches begin to freeze solid. By the last week of November the entire river is frozen; the upper reaches remain frozen for some 150 days, the lower for 220. The thawing of the ice—which takes longer than the freezing—lasts from the end of April (upstream) to the end of May, and the spring drift (about five days in duration) produces considerable ice jams. The difference in level between high water and low is 25 feet (8 metres) at Novosibirsk on the upper Ob; it reaches 43 feet (13 metres) at Aleksandrovskoye on the middle Ob but decreases to no more than 20 feet (6 metres) at Salekhard near the mouth. The water is warmest in July, reaching a maximum of 82 °F (28 °C) in the vicinity of Barnaul.
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