Topographic (topo) maps are used to define watershed boundaries. By connecting the highest points surrounding a creek or other body of water, you can outline the boundaries of a watershed on a topo map.
A topographic map shows differences in elevation using a series of contour lines. You can think of a contour line as a trail for a lazy hiker who never wants to climb up or down. Instead, he just walks around the side of a hill at the same elevation.
On topo maps, the change in elevation between contour lines is usually
forty feet. So the lazy hiker on one contour line is forty feet higher
or lower in elevation than the lazy hiker on the neighboring contour line.
Hilltops are easy to find on topographic maps because they
are shown as closed circles. The lazy hiker can walk around and around
the top of the hill on the same elevation. In contrast, streams are
usually shown as V's with the point of the V being the upstream end.
When examining a topo map, you will need to orient yourself
by finding known objects. Look for peaks (which are often labelled
by name), buildings (shown as small squares), churches, graveyards, etc.
Labelled roads can also help you get your bearings.
Once you are oriented, find the point you are interested in. Continuing with the sample map shown above, I have highlighted Creek X. Now you can begin to define the area's watershed boundaries.
Starting at the creek, go uphill in every direction (except directly downstream) until you reach the highest points. Sometimes the highest point above the creek may be a peak, shown as a closed circle. Other times, the highest point is a ridge, shown as an elongated U- shape. In any case, on the other side of these highest points, a new watershed begins. Connect the highest points around the creek with a line and you have drawn in the watershed boundary. This line also marks the headwaters of the watershed.
You can find topo maps of most areas in the United States at http://www.topozone.com/.