Cartographic problems in stream channel delineation

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Abstract
The acquisition of data for stream network analysis is largely dependent on the availability of topographic maps and the accuracy with which data is portrayed thereon. When Yoxall (1969) examined the 1:250,000 series of maps of Ghana, he found inconsistencies in the selection of rivers for mapping. Scheidegger (1966, p. 57) reported that the blue stream lines printed on maps are extremely arbitrary and appeared to depend on the channel flow observed to be present by the surveyor or by means of air photographs. When Morisawa (1957) mapped the channel system of a part of the Appalachian Plateaus in spring she found much more running water in channels than was depicted on the U.S. Geological Survey 1:24,000 topographical maps, even when they were produced from air photographs, it being particularly apparent that many flowing fingertip tributaries had been obscured from the air by overhanging trees. The policy of the U.S. Geological Surveys has been to classify all streams as either perennial (i.e. ones that flow throughout the year) or intermittent (i.e. ones that are dry for a considerable time each year, say for three months or longer). Before criticising such a classification as over-simplistic, it is pertinent to consider the problems in identifying a stream channel network in the field and on maps and air photographs.

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