Continental Divide of the Americas

• 1. Introduction

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  The Continental Divide of the Americas is a line that splits a hemisphere. Rain falling on one side of it will eventually reach the Pacific Ocean. Rain falling on the other side drains toward the Atlantic, the Arctic, or the Gulf of Mexico. That single fact, which sounds almost too simple to be true, organizes the movement of water across two continents.

  The Divide stretches from the Bering Strait in the north all the way to the Strait of Magellan at the tip of South America. It is the principal hydrological divide of the Americas, and it is largely mountainous. For much of its length it follows the high peaks of the Rocky Mountains and the Andes, sitting at elevations that dwarf the other water divides of the Americas.

  But the Divide is not a clean, simple line. There are basins along its route where water goes nowhere at all. There are lakes that drain in both directions simultaneously. There is a single peak in Montana where three oceans share a drop of rain. What lies ahead is a journey across the full sweep of this extraordinary feature, from the Arctic shore of Alaska to the frozen shores of Tierra del Fuego.

• 2. A Line Through Two Continents

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  Cape Prince of Wales, the westernmost point of the Americas, is where the Continental Divide's geographic path begins. From there it moves through Arctic Alaska, reaching its northernmost point near the Beaufort Sea, close to the U.S.-Canada border. The route then bends south, crossing the Yukon and tracing part of the boundary between the Yukon and the Northwest Territories through the Mackenzie Mountains.

  The Divide moves into the Northern British Columbia Interior, threading the Cassiar Mountains, the Omineca Mountains, and the northern Nechako Plateau before arriving at Summit Lake, just north of the city of Prince George. From there it crosses the McGregor Plateau to the spine of the Rockies, following the crest of the Canadian Rockies southeast to the 120th meridian west, where it marks the boundary between southern British Columbia and southern Alberta.

  At the U.S. border, the Divide enters Montana at the boundary between Waterton Lakes National Park and Glacier National Park, bisecting Glacier before heading south toward Helena and Butte. It passes through the namesake community of Divide, Montana, continues through the Anaconda-Pintler Wilderness, and eventually crosses into Wyoming within Yellowstone National Park. From there it loops around the Great Divide Basin and into Colorado, where it reaches its highest point in North America at the summit of Grays Peak, standing at 4,352 meters.

  South of Colorado, the route crosses U.S. Highway 160 at Wolf Creek Pass, where a line on the ground marks the division. It then descends into western New Mexico before entering Mexico, passing through Chihuahua, Durango, Zacatecas, Aguascalientes, Jalisco, Guanajuato, Querétaro, México state, the Federal District, Morelos, Puebla, Oaxaca, and Chiapas in turn. Entering Central America, the Divide passes through southern Guatemala, southwestern Honduras, western Nicaragua, western and southwestern Costa Rica, and southern Panama, reaching its lowest natural point at the Isthmus of Rivas in Nicaragua, just 47 meters above sea level. The Panama Canal cuts through it at 26 meters.

  In South America, the Divide follows the peaks of the Andes, traversing western Colombia, central Ecuador, western and southwestern Peru, and eastern Chile, essentially following the Chile-Bolivia and Chile-Argentina boundaries. It passes through the Punta Cancanan Pass in Peru at 15,420 feet before ending at Cabo San Diego at the southern tip of Patagonia and Tierra del Fuego.

• 3. Triple Divide Peak

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  Triple Divide Peak in Glacier National Park, Montana, holds a distinction that no other spot on Earth can claim. It is where the primary Continental Divide and the Northern or Laurentian Divide converge. Water that falls on its slopes flows in three separate directions: toward the Pacific Ocean, toward the Atlantic Ocean by way of the Gulf of Mexico, and toward the Arctic Ocean by way of Hudson Bay.

  Most geographers, geologists, meteorologists, and oceanographers consider Triple Divide Peak the hydrological apex of North America. That assessment depends on how Hudson Bay is classified. The International Hydrographic Organization, in a working edition of its Limits of Oceans and Seas document, defines Hudson Bay as part of the Arctic Ocean, specifically labeling it "Arctic Ocean Subdivision 9.11." The Bay's outlet runs from 62.5 to 66.5 degrees north, just south of the Arctic Circle.

  No other place on any continent matches this. Only Asia borders three oceans, but an enormous endorheic basin stretching from western China to the Aral and Caspian Seas separates any Arctic and Indian Ocean tributaries by vast distances. North America's triple-ocean drain point at Triple Divide Peak is, by that measure, unique in the world. The designation of Waterton-Glacier International Peace Park as the "Crown of the Continent" is tied directly to this status.

  Not everyone accepts that Hudson Bay is part of the Arctic Ocean. The Bay sits entirely south of the Arctic Circle, and its water budget connects more to the Atlantic than to the Arctic. Baffin Island largely cuts off Hudson Bay from Arctic channels, so much of its water mixes with the Atlantic via Hudson Strait rather than flowing north. The ice that travels down the Saskatchewan Glacier, for instance, ultimately reaches the Atlantic.

  If Hudson Bay belongs to the Atlantic, the triple-ocean point shifts to a very different location: a snow-covered hump on the border between Alberta and British Columbia, on the southern slope of Snow Dome at 3,456 meters. The Columbia Icefield sits on top of it, and the exact point shifts from year to year because the snow, about 10 meters annually, does not flow as water but creeps downhill as glacial ice. That ice divides three ways: down the Athabasca Glacier to the Arctic via the Athabasca and Mackenzie rivers, west to the Pacific via Bryce Creek and the Bush and Columbia Rivers, and down the Saskatchewan Glacier to Hudson Bay via the North Saskatchewan, Saskatchewan, and Nelson rivers.

• 4. Secondary Divides and Triple Points

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  North America has five major drainage systems: the Pacific, the Atlantic, the Arctic, Hudson Bay, and the Gulf of Mexico. Some sources, including the International Hydrographic Organization, add a sixth in Canada's Northwest Passage basin. Using just the five primary systems, four secondary continental divides and three secondary triple points exist across the continent.

  The Arctic Divide, also called the Northern Divide, separates the Arctic Ocean watershed from the Hudson Bay watershed across northern and western Canada. The Laurentian Divide, sometimes also called the Northern Divide, further separates the Hudson Bay-Arctic Ocean drainage region from the Atlantic watershed region. Secondary divides also separate the Great Lakes and Saint Lawrence River basin from the Gulf of Mexico drainage via the Missouri-Mississippi-Ohio river system. A divide along the Appalachian chain separates streams flowing directly to the Atlantic from those that reach it via the Ohio and Mississippi Rivers.

  Two of the three secondary triple points have already been noted. The third sits near Hibbing, Minnesota, where the Northern Divide intersects the Saint Lawrence Seaway divide. There is no firm consensus on the definition of a continental divide, so there is also no agreement on precisely where these secondary points fall. The main Continental Divide, however, is a far more distinctive geological feature than all of them.

• 5. Basins, Lakes, and Exceptions

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  Several endorheic basins complicate the otherwise clean picture of water flowing either east or west. An endorheic basin is one where water drains inward and does not reach any ocean. Notable examples straddling or adjoining the Continental Divide include the Great Divide Basin in Wyoming, the Plains of San Agustin and the Animas Valley in New Mexico, the Guzmán Basin in New Mexico and Chihuahua, Mexico, and both the Bolsón de Mapimí and the Llanos el Salado in Mexico.

  The standard method for assigning such basins to one side of the Divide is called the overflow rule: the basin is assigned to whichever side it would drain toward if progressively filled with water until it overflowed. Large-scale maps often show double divide lines when endorheic basins are present. The detailed USGS topographic maps of the United States, however, generally show only the main Divide as determined by this rule, which avoids the need to trace boundaries for very shallow basins with poorly defined rims, such as the San Luis Closed Basin in Colorado or the basin of the lost streams of Idaho.

  Even rarer exceptions occur where a stream near the Divide splits and simultaneously flows in both directions, or where a lake straddles the Divide and overflows toward both oceans. North Two Ocean Creek and Isa Lake, both in Wyoming, are examples. The Panama Canal shares this feature, though by human design. Both the Chagres and Gatun rivers flow into Gatun Lake, which empties to both the Pacific and Atlantic Oceans.

  Several small lakes in the Rocky Mountains between Alberta and British Columbia also drain into both provinces and thus into both the Arctic and Pacific Oceans. One is the "Committee's Punch Bowl," a small lake in the Athabasca Pass. Another well-known case is the Abbot Pass Hut of the Alpine Club of Canada, which once sat directly on the Divide at Abbot Pass on the boundary between Banff National Park and Yoho National Park. Precipitation falling on the eastern half of its roof drained via Lake Louise into Hudson Bay. Rain on the western half flowed via Lake O'Hara into the Pacific.

• 6. Hiking the Divide

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  The Continental Divide National Scenic Trail, known as the CDT, follows the Divide through the United States from the Mexico-U.S. border all the way north to the Canada-U.S. border. The trail is not a single dedicated footpath but a corridor of pathways, combining dedicated hiking routes and back roads that run on or near the Divide.

  North of the border, a less-developed extension called the Great Divide Trail picks up where the CDT ends. It continues through five national parks and six provincial parks in Canada, ending at Kakwa Lake in east-central British Columbia, at the edge of one of the most remote stretches of the Divide's entire length.