Book Excerpt : Understanding the Continental Divide

Driving between Old Faithful and West Thumb, you twice pass signs identifying the Continental Divide. This makes it difficult to understand the concept of the divide or to visualize where the water is going. Maps show the Continental Divide as a thin line crossing irregularly from the park's western boundary to leave the park near the southeast corner. What does this line represent?

A divide is a geographer's term for the topographic boundary between adjacent drainage basins or watersheds. Runoff from precipitation that falls on one side of the divide will find its way into one stream system; on the other side of the divide, surface runoff discharges into another stream system.

The Continental Divide, (also known as the Great Divide) is the main watershed boundary of the North American continent. As shown in Figure 3, the divide separates areas that drain to the Pacific Ocean (green) and the Atlantic Ocean (lighter green). The area around Old Faithful Village drains to the Atlantic Ocean via the Firehole River, a tributary of the Madison River. About 9 miles east of Old Faithful, the Grand Loop Road crosses the Continental Divide at Craig Pass, entering the watershed of the Lewis River, which flows to the Pacific Ocean via the Snake and Columbia rivers. A raindrop falling on one side of Craig Pass may be destined for Astoria, Oregon, while a drop falling a few feet away on the other side of the pass could meet the Gulf of Mexico near New Orleans.

Figure 3. Map of stream flow directions and the Continental Divide.

Isa Lake is situated right across this line at Craig Pass. This is a rare type of place where the top of the divide is actually flat enough to permit a small depression where a pond can form. Looking at the lake, you would think that the water from its southwestern outlet would flow to the Pacific and that from the northeastern outlet to the Atlantic, but this is not so.

In this area the Continental Divide doubles back on itself and the road crosses it again, as you can see from the sketch. Using the old name, continental watershed, might make it easier to visualize.

—With contributions by Linton A. Brown

* * * * *

 This is an excerpt from Yellowstone Treasures, 2nd edition, pages 107–8.

Copyright 2005. All Rights Reserved.