Isa Lake may appear insignificant except during the brief summer months when yellow pond lilies carpet its surface, but spring runoff can swell it beyond its tiny borders and make it do something extraordinary: it drains toward two oceans, and does so in reverse of what might be expected, with its east drainage flowing into the Pacific and its west drainage beginning the long journey to the Gulf of Mexico. This unusual behavior lies along the Continental Divide, an imaginary line running through the mountains from Alaska to Mexico that separates waters flowing to the Pacific Ocean from those flowing to the Atlantic Ocean. For early travelers, this mountain backbone of the continent presented an immense obstacle that could demand days or even weeks of travel.