A few weeks ago, I was asked to write about the continental divide. I have been thinking (and researching) an answer in the interim.
There is a very simple answer to the question of what causes a continental divide and that is "gravity." Water can only flow downhill and it will flow from the highest point to the lowest point. For most water, this gradient generally means from the peaks of mountains to the oceans.
This is the hydrogeological image we all had in our elementary school books about the water cycle.
Water evaporates from the oceans, collects in clouds, travels over the land, falls as precipitation (snow, rain, hail, or fog) upon the land and mountains, and is returned to the ocean by the myriad of streams and rivers that cross the intervening country.
This cycle replenishes the freshwater we find on land and ultimately means the oceans are the reservoirs for the vast majority of Earth's water - about 97 per cent. Indeed, freshwater makes up only 2.5 per cent of the total and about 98 per cent of that is tied up in either glaciers and the ice caps or groundwater.
The flow of water is a little more complicated than the simple water cycle portrays but essentially there is a constant flux between the various reservoirs of water on Earth with the oceans being the ultimate sinks for all water on the planet.
A continental divide occurs because the path for water is generally downwards to one of the oceans.
As North America is surrounded by three separate oceans, water can only go to one of them and so there are actually several "continental divides" separating waterways flowing to the Pacific, the Atlantic, and the Arctic. If we include Hudson Bay and the Gulf of Mexico, where water flows and how it is divided between the various regions is quite complex.
For example, the continental divide between the Pacific and Arctic oceans starts in Alaska, passes through the Yukon, and then zig zags its way to Summit Lake just north of Prince George before traversing the McGregor Plateau and the spine of the Rockies. Water south of Summit Lake flows to the Pacific while water to the north flows through Crooked River into the Peace and ultimate to the Arctic Ocean.
Triple Point Peak in Glacier National Park is the point where the principal North American divides converge with water flowing to the Pacific Ocean, the Atlantic Ocean via the Gulf of Mexico, and the Arctic Ocean via Hudson Bay. Alternatively, if you consider the Hudson Bay to be part of the Atlantic (and some geographers do) the three divides meet at Snow Dome - an unassuming hump of snow on the Alberta/B.C. border near the Athabasca Glacier.
But why is the continental divide in the Rockies? How did it arise?
The answer is plate tectonics and the shaping of the surface of the Earth.
The Rocky Mountains are a fairly old mountain range resulting from the collision of the North American plate with the vastly larger Pacific Plate.
As continental plates are less dense, they tend to ride up and over the ocean floor. The result is a subduction zone down the entire west coast of the Americas where the Pacific plate submerges back into the mantle. This fault line gives rise to deep sea earthquakes and will ultimately cause havoc for our coastal cities.
The re-melting of the ocean's crust gives rise to the Cascade volcanoes which stretch from southern Washington to well up the B.C. coast. But the coastal mountains are much younger than the Rockies themselves. Presently, geologists believe the Rockies resulted from an inherent fault line in the continental crust which flexed as the plates first met.
The Rocky Mountains were once the floor of a vast interior sea that covered much of the interior of North America. As the plate was put under pressure, this region started to lift as evidenced by the fossil record which shows sea dwelling creatures in the mountain rock strata. One of the most famous of these is the Burgess Shale outside of Field, B.C.
As the crust lifted, it also broke and the resulting ridges forced water to flow in one direction or the other. Long before the Rocky Mountains reached their present heights, the pathways for rivers such as the Fraser and the Saskatchewan were being laid down.
As the land continued to change and deform, generating our present Rocky Mountain range, precipitation in the mountains established paths to the sea. These have been affected over time by climatic changes, such as glaciation, but the one thing is for certain - some of the water goes west to the Pacific, some north to the Arctic, and some finds its way east to the Atlantic.
As long as the Rocky Mountains stand, there will be a continental divide in North America.
