Larry Jackson, age 14, of Salt Lake City, Utah, for his question:
What causes the Great Divide?
A continental divide may be called a watershed. It is a subtle geographical feature that drains the runoff from precipitation and launches young streams in opposite directions. North America's major watershed is called the Great Divide, perhaps because it is long enough to extend from the northern Rockies down through Central America. It works with no effort at all, simply because water is obligated to flow down even the gentlest slope.
North America, like most continents, has an enormous central region of flat plains, with mountainous ridges on both sides, sloping down to major coastlines facing the oceans. A map based on altitudes shows its outstanding ups and downs in greens and browns and we can expect the main rivers to drain downhill from the slopes to the plains. So they do. But as a rule, the map reveals these geographical features only in a general way. It fails to note that the entire land mass has long, though very gentle humps that tend to tilt portions of the continent in various directions.
These ridges are the continental divides that separate the streams and cause them to flow in opposite directions. Naturally, these humps are on high ground. But as a rule, their summits and slopes are very subtle and hard to survey. They are watershed regions where rains and melting snows feed streams starting out on their journeys downhill to the seas. One way to pinpoint such a watershed is to trace a stream uphill to its source. When the headwaters of another nearby stream start to flow in the opposite direction, you have found the summit of a continental divide.
The summit of the Great Divide runs through Glacier National Park. At Cutback Pass, the runoff drains into three streams, flowing in different directions. Their headwaters are so close that a person can reach over and pour a pitcher of melted snow into each of them. The Great Divide runs north and south, separating, the runoff to drain downhill in opposite directions. As you would expect, the streams born in this watershed are led either eastward or westward.
However, other ups and downs along their courses may veer their channels in other directions. For example, those three neighboring streams born in Cutback Pass have very different destinations. One veers northward to empty its waters in Hudson Bay; another veers southward and finally reaches the Gulf of Mexico. The third runs westward and dumps its drainage into the Pacific Ocean. The streams divided by a watershed naturally gather others down the gentle slopes of the continent and usually reach the seas as surging rivers.
The Great Divide is on high ground, though it meanders around the towering peaks. Another divide runs east and west. It is just high enough to separate streams flowing in more or less north and south directions. The headwaters launched down the northern slopes may be veered to the Gulf of Saint Lawrence, to Hudson Bay or to the Arctic Ocean. Most of those guided down the opposite slopes run to join the mighty Mississippi on its way to the Gulf of Mexico.