Twigs and blades of grass exist atop the Continental Divide, little more than a dozen miles from my house as the crow flies, that might split a raindrop in two and send the two halves to oceans separated by thousands of miles of land. One half will flow down the west slopes of the mountains and fall into rivers flowing westward to the Pacific Ocean, some 600 miles distant. The other half of the raindrop will flow down the east slopes and find a way to the Gulf of Mexico and the Atlantic Ocean, some 3,700 miles to the southwest by way of river routes. The lofty and unbroken elevations of the Rocky Mountains (reaching from Mexico all the way through the United States to Canada) separate the direction of flow for the watersheds on each side of the range.
There also exists, in Montana’s Glacier National Park, a mountain named Triple Divide Peak (8,020 ft.) which sheds water into three distinct continental drainages. The water shed from the peak can potentially flow to the Gulf of Mexico by way of the Missouri; north to The Hudson Bay, by way of the Saskatchewan River Basin; or to the Pacific by way of the Columbia.
Elevation also plays a more linear role in separation. The simple act of driving near a mountain range here in Montana today will reveal the distinct, and currently receding, snow lines on the higher peaks. Spring ends at the snow line. As spring turns to summer, the wide valleys tend to dry and the green gradually recedes to higher (cooler and wetter) elevations. According to the USDA, Montana supports seven plant hardiness zones—largely based on elevation and weather alterations created by the presence of mountains.
Some lines of elevation are firm. The tree line here in Montana, for example, is somewhere just above 8,000 feet. The climate is simply too harsh above this elevation to support trees. The higher elevations tend to be either jumbles of stone and snow or alpine tundra. Taking elevation in the other direction, mountain huckleberries do not naturally grow in elevations below 2,000 feet.
The death zone begins at 26,000 feet. Life cannot be supported above this elevation. Essentially, climbers challenging Everest begin dying once they ascend above this elevation. The trick is to reach the top, take a look around, and scramble back down to an elevation below 26,000 feet before death catches them. The air is far too thin and cold in the death zone. Additionally, the UV radiation found in sunlight (no longer naturally filtered by the atmosphere) will blind climbers without protective eyewear. Similar to Icarus in Greek Mythology, we cannot climb too near the sun without suffering the consequence of death.
Elevation cannot be denied. Standing at my bay window today, I can look across the valley and see the snow on the upper quarter of the Elkhorn Range. Below, I see the jade-color of spring’s pine forests. The forests eventually spill down onto grassy scarps and end there. The scarps then flow down onto the prairie on which I live—happy to challenge the huckleberry line and the tree line now and then.
Sources: New World Encyclopedia, National Parks Service, Wikipedia, USDA, U.S. Forest Service, University of Idaho