WASHINGTON — For an hour or so Greenland had its own mighty waterfall, flowing secretly at three times the volume of Niagara. A meltwater lake on the surface of a glacier suddenly emptied in July 2006, sending millions of gallons of water through cracks in the ice sheet to the ground where it could affect the movement of the ice. The lake covered 2.2 square miles near the western edge of the ice sheet and took about 24 hours to drain. During the most rapid 90 minutes, water was flowing out of the lake at a rate of 2.3 million gallons per second, according to researchers led by Sarah Das of Woods Hole Oceanographic Institute in Woods Hole, Mass. Under international convention, the minimum flow of Niagara Falls in summer is about 750,000 gallons per second. The findings are reported in a pair of papers about the Greenland ice sheet appearing in Thursday’s online edition of the journal Science. Das and Ian Joughin of the University of Washington in Seattle led the teams that produced both papers. ‘We found clear evidence that supraglacial lakes - the pools of meltwater that form on the surface […]