|Diablo Reservoir below the peaks of North Cascades National Park. The Turquoise color results from fine silt and clay derived from glaciers.|
The ice ages put an end to that. A huge lobe of ice flowed south out of Canada, blocking the flow of the upper Skagit. The river backed up into a large lake, and eventually the lake breached the rim at a different location and started flowing west. The meltwater from the glaciers added to the ability of the river to carve downward, and the Skagit River Gorge was quickly cut (at least in geological terms). The river that flowed west was now many times larger, and the Skagit Gorge was too much of a temptation for any dam engineer to resist. Three reservoirs were built, Gorge, Diablo and Ross, and today they provide around 20% of the electricity used in Seattle.