A torrent frozen in place
The Lena River, one of the world's longest, flows for 4,400 kilometers from its source near Lake Baikal northward to the Arctic Ocean. For up to eight months of the year, vast stretches of the river are frozen solid, with ice thickness reaching over two meters. The annual spring breakup is a massive event, driven by a simple geographic fact: the river's southern upper reaches thaw while its northern mouth is still locked in Arctic ice.
Starting in late April or early May, meltwater from the vast 2.5 million square kilometer drainage basin begins to move. This surge of water lifts and shatters the immense ice sheet, creating a procession of grinding, fracturing ice floes. This moving front of water and ice travels downstream, its speed dictated by temperature and the volume of meltwater. The process releases about 40% of the river's total annual discharge in just a few weeks. During this peak, the flow rate can be 65 times greater than it is in April. The force of the moving ice reshapes the riverbanks, uprooting trees and scouring away large sections of shoreline.
The catastrophic power of ice jams
The most dangerous phase of the breakup is the formation of ice jams, known locally as zators. These occur when the downstream progress of broken ice is blocked by the still-frozen sections of the river to the north. Water and ice pile up behind this dam, causing the river level to rise with astonishing speed.
The city of Lensk has experienced some of the most destructive floods. In May 2001, a massive ice jam caused the river to rise to 19.5 meters, six meters above the critical point. The flood submerged thousands of homes, left 14,000 people homeless, and required the evacuation of most of the town's 27,000 residents, many rescued by helicopter from their rooftops. To combat the most severe jams, Russian authorities have used military jets to drop bombs, physically blasting apart the ice to release the pent-up water. In the 2001 Lensk flood, Su-24 bombers were deployed to break a jam that stretched for miles. These events are not isolated. The unique south-to-north flow of Siberia's major rivers makes them prone to this type of catastrophic spring flooding.