A glacier-dammed lake is emptying into the Salmon River near Hyder

High above Hyder, a lake is draining out the bottom of a glacier — and the Salmon River is rising to take it. The outburst from Summit Lake began Monday, and the National Weather Service says the road from Hyder to Premier, in Canada, could flood near mile 9 as the river climbs.

Summit Lake is not an ordinary lake. It's the largest self-draining glacier-dammed lake in Canada, held in place by the Salmon Glacier. All year it fills with rain and meltwater, until the water floats and undermines its own ice dam and pours out beneath the glacier — a jökulhlaup — dragging icebergs off the dam as it goes. When it lets loose, the Salmon River can roughly triple in flow, and it has washed out the Premier road before.

It has done this nearly every summer since 1961, when the lake first drained abruptly and tore up the valley. The trouble is that no two years are the same. "It is unknown how much water will be released from Summit Lake," the NWS said, which leaves the crest height and timing uncertain. The river is expected to peak Wednesday or Thursday.

Anyone near the water should stay clear: the flood carries debris, and it's cold enough — straight off the glacier — to bring on hypothermia fast. Despite the yearly drama, the lake has actually been shrinking over the decades, which points toward smaller floods over time, not bigger ones.