Two engines struggle up the last few feet to the summit of Montana's Mullan Pass in a driving rain. Behind them is the Missouri River. When the nose of the lead engine tips over the top and points downhill, it will be headed for the Pacific Ocean. Consider the rain. Two drops leave the clouds simultaneously, one lands an inch east of the divide, bound for the Gulf of Mexico, one lands an inch west, bound for the Pacific. The Great Divide, where two inches turn into 2000 miles.