How to Compare a Drainage Divide and a Watershed

Understand the meaning of the terms in a North American context. A surface watershed here refers to a drainage basin itself: the area drained via runoff and overland flow (in streams, creeks and rivers) to a common outlet. (Keep in mind that groundwater, an important part of the water cycle, may or may not correspond in its movements to surface watershed boundaries.) A watershed can be taken on different orders of magnitude: It could restrict itself in meaning to the little coulee drained by an ephemeral creek in a tract of Great Plains badlands, or to the entire Missouri River system of which that coulee creek is a tiny component. Drainage divides separate different watersheds: They are the high points that serve as boundaries between multiple basins. Again, they may be small or large, ranging in size to those continental divides that separate massive areas outflowing to separate oceans.

Look at a topographic map to get a deeper sense of the concepts. This will require a basic understanding of contour lines, which are the squiggly marks on such a map that signify positions of common elevation. Depending on the map's scale, it will show contour lines of different intervals: A 400-foot interval means that 400 feet separate any two consecutive contour lines. While usually not designated on a topographic map, drainage divides show up as high plateaus or ridges off which streams (typically shown in blue) tumble. In highly rugged country, divides may be knife-thin ridges cast between steep canyon walls, but in a swath of gentler landscape -- the rolling glacial plains of the Central Lowlands, for example -- a drainage divide may be a subtle swell of moraine or drumlin.

Investigate the divides and basins in your area. Even in the heart of a huge city, you belong to a watershed. Find out your immediate drainage, even if it's a channelized stream or underground conduit, and learn where those waters end up. Look for your home's drainage divides: In a rural setting, it might be a cliff-edged mesa, while in an urban lowland it might be the rolling hills of an affluent neighborhood. While hiking or backpacking, always keep an eye to the different sub-watersheds you're traversing, and make note of the divides: Differing micro-climates in one basin vs another due to environmental aspects like slope and aspect can suggest the complexity of ecological systems.