Many rivers in the Eurasian and North American continents flow northward and empty into the Arctic Ocean. In descending order of annual flow, the four largest of these rivers are the Yenisey River, with the seventh largest annual flow in the world, and the Ob, Lena and Mackenzie Rivers. Although documented estimates vary widely, the total annual flow into the Arctic Ocean is thought to be in the neighborhood of 2,500-3,500km3. Because the surface water in the Arctic Ocean flows into the Atlantic Ocean on the Greenland Current, the Arctic Ocean is believed to form a vast reservoir of river water supplying the Atlantic Ocean. The annual water volume supplied from the Arctic Ocean to the Atlantic Ocean is the world's second largest after that of the Amazon River.
Annual flow of major rivers into the Arctic Ocean (Aagaard-Carmack, 1998)
These rivers are covered in thick ice in the winter. In the spring, the snow and ice upstream, which is further south, begins to melt, carrying the runoff downstream. This runoff collects and swells in the lower reaches, lifting and crushing the river ice and carrying it further downstream to the area where the ice is thickest, forming ice jams. Pent up by the ice jam, the water overflows its banks and changes course. These natural processes in the Arctic have left their traces everywhere across the vast tundra in the form of meandering rivers and unique semicircular lakes.
The volume of river flow into the Arctic Ocean is highest during the spring thaw, exhibiting dramatic seasonal changes. In June, when the monthly flow volume peaks, the runoff of the Yenisey and Lena Rivers reaches 80,000 metric tons of water a second, while the Ob discharges 30,000 metric tons and the Mackenzie 15,000 metric tons. In the winter, these rivers contract to a relative trickle of 5,000 metric tons of water per second - a mere fraction of their springtime height. Even at these seasonal lows, however, these great rivers are far larger than even Japan's major rivers at their maximum flow volumes.
Ice jam (Photo: National Institute of Polar Research)
Seasonal variation in flow of the four largest Arctic rivers (Carmack, 1986)