Big Ice

Richard Harris accompanied the team during part of this trip to produce radio stories for NPR. While satellite observations showed that lakes could drain over the course of a day, the pressure sensors revealed the startling fact that the North Lake had drained in about 90 minutes. When it drained, the lake was nearly 4-km (2.4-miles) long and 40-feet deep, which meant that water drained into the 3.2-km (2-mile) long crack in the ice at a rate that exceeded the flow over Niagara Falls. Needless to say, work with boats on these lakes has been discontinued and instruments are now placed and retrieved when the lakebeds are dry. During the drainage event, the GPS recorded surface uplift of 1.2 m (4 feet) as the water filled the space under the ice sheet, confirming theoretical predictions that water could fracture through thick ice. Once the lake drained, the pressure in the thick ice closed much of the long crack through which the lake had drained. In a few isolated spots where melt streams continued to flow into the crack, melting was able to offset the tendency for pressure to close the crack, creating conduits to the bed called “moulins” that last through the rest of the melt season. The team also discovered that the South Lake and another lake just to the north of North Lake were draining more slowly through moulins that had formed where the lakes overflowed into crevasses along the shoreline. When melt water filled these crevasses, they fractured parallel to the shoreline rather than through the lake bottom. As a result, the overflow from the lakes had to slowly melt a deep channel over several weeks to drain all the way to the bottom.