Keep firmly in mind that in a valley, the contours tend to “vee” up the valley.Here are some points that should help you in tracing stream courses where there are valleys with no stream shown in them. (Remember about the “V”s of the contour lines pointing upstream in valleys.) You yourself can trace the courses of such streams by recognizing the position and downslope direction of such valleys. So it’s just a matter of extending the streams shown on the map farther upstream, to where valleys are no longer defined and the land surfaces slopes uniformly. These are usually located in the headwaters of larger streams, which are shown on the map. Many well-defined valleys, however, which presumably would have streams flowing in them briefly after a heavy rain, have no streams shown in them. Permanent streams are always shown as thin blue lines on official topographic maps, and ephemeral streams (those that flow only after a heavy rain) are often shown as dot–dash blue lines. In some places streams “expand” into lakes, but the principle is the same. In most areas of the world, except in the driest of deserts (and beneath glaciers), one can trace fairly easily on a topographic map the system of main streams and their tributaries.
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