Mapping Your Way to a Better Trip

By Mike MacLeod

Maps can obviously tell us where to access a local river by showing where roads and trails intersect its course. But maps offer much more.

A river's gradient is of primary concern when planning a river trip. With a topo map's evenly-spaced contour lines, it is easy to compute a river's gradient by dividing the difference between two contour lines by the distance in river miles between them.

For instance, if one contour reads 1,760 feet above sea level and the next downstream contour line reads 1,740, and it is a mile downstream, the gradient is (1760-1740)/1=20 feet/mile.

So what does that say about the character of the river? Below six feet/mile, a river will have very little whitewater. Of course, the closer this value is to zero, the more lake-like paddling you will experience. A blackwater river will exhibit a low gradient as well as serpentine curves, and its banks are usually marked with swamp symbols. Oxbow lakes may also occur along its course.

Waterfalls appear on topo maps as a convergence of two or more contours on a river, though keep in mind that if a waterfall is significantly less than the contour interval, it may not show up at all. Contours also show where a river is pinched by the surrounding terrain, indicating a gorge area where the river flow will invariably speed up. Here, you may have to contend with drops and boulder fields.

For extended trips, topo maps are valuable for picking out a campsite in advance. Look for a flat area indicated by widely-spaced contour lines that are high enough above the waterline to prevent flooding. The view from camp, drinking water sources, and even orientation to the morning sun for drying tents can be considered before the trip begins. Often, a good campsite may not be visible from the river, but are evident on a map.

Finally, topo maps may warn of man-made dangers, such as dams (revealed by converging contour lines and their upstream lakes), pipe lines, and water treatment facilities (circular or rectangular ponds). Converging power lines may indicate a power plant, and the word "stack" will indicate smoke stacks. Many power plants have small dams that diverts water into their cooling intakes. Keep in mind, however, that 1928-vintage USGS topo maps still exist and may poorly represent modern conditions of a riverway. Luckily, residential areas are more likely to be updated than rural areas, though well-known wilderness areas often have commercially-available maps that far exceed the detail of USGS maps.

Topo maps may be obtained for most areas from the United Sates Geological Survey. In addition, larger universities are often map depositories and will have most topos of the United States and Canada that have been published. Call their geology department about using these maps for reference. Aerial photos, some with contour lines drawn in, may be available from your local water conservation district or county planning office. With a good magnifying glass, you may be able to spot every rapid on a river.

Also see:
The River Next Door

Published: 29 Apr 2002 | Last Updated: 15 Sep 2010
Details mentioned in this article were accurate at the time of publication


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