A Morphometric Analysis of Some Piedmont Sub-Regions

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A MORPHOMETRIC ANALYSIS OF SOME PIEDMONT SUB-REGIONS* J a m e s F. W o o d r u f f University of Georgia In 1925 LaForge described in general qualitative terms the terrain differences of the Georgia Piedmont.1 He recognized two major divisions Piedmont Georgia and Midland Georgia and further subdivided these into three and four sections respectively. Lacking adequate topographic maps or aerial photo coverage La Forge’s delimitation, of necessity, was Based largely on differences in relief or elevation and on degree of dissection either apparent or intuitively recognized. Throughout his descriptions runs a rather plaintive note of frustration with the in­ adequacies of information and techniques for determining boundaries. This paper attempts to verify the reality of three of his subdivisions by applying some of the quantitative techniques more recently developed. The physiographic units thus tested are the Washington Plateau, the Midland Slope, and the Atlanta Plateau (Figure 1). The distribution of test areas was purposely aligned in a N.W.-S.E. direction parallel to the general Piedmont slope in order to test the assumed change in stage of erosion with change in elevation. The actual selection of test drainage basins was dependent, unfortunately, upon availability of air photos. This limitation undoubtedly casts some doubt upon the reliability of certain statistical results as does the relatively small sample of basins tested— twenty-five. PHYSIO G RAPHIC S U B D IV ISIO N S OF T H E GEORGIA P IE D M O N T go 0 SO IQ OM it?* ail*r LO fere? FIGURE I 17 Recognizing the wide variation in size of drainage basins but the validity of comparing basins of the same order as classified by Horton2 and Strabler3, all tests were confined to basins of third order or less. Parameters measured were both dimensionless and dimensional with the latter including linear, aerial and hypsometric properties of drain­ age basins. Plotting the mean length of first, second and third order streams of test basins for the three areas indicates that the increase in stream length of successively higher orders is a geometric progression and thus in accord with the general law of stream length as stated by Horton4, (Figure 2 ). The slope of the line, is somewhat less for the Atlanta Plateau than for the other two test areas. It is apparent that streams of all orders in the more southerly Washington Plateau and Midland Slope have longer mean lengths than those of the Atlanta Plateau. Calculating length ratios between second and third order streams indicates that second order streams increase their length at an accelerated rate with each successive area southward, bearing out the decreasing angle of slope on these graphs. This fact corresponds nicely with a postulated increase in maturity of dissection as the Coastal Plain is approached. The slight differences in mean length of streams and in length ratios between the Midland Slope and the Washington Plateau alone, however, would not seem to warrant separating these areas into two distinct sections. As stated by Horton5 and verified by Schumm6 and others, as stream basin development progresses, angles of junction and bifurcation of tributary streams should decrease in response to a general lowering of the watershed and a corresponding decrease in difference between stream gradient and upland slope. In the Piedmont test areas, therefore, angles of confluence and bifurcation should decrease southward from the Atlanta Plateau. Unfortunately, within the test areas, there is no con­ sistency in change of these angular attributes from section to section. Mean angle of junction of first order channels increases southward while mean bifurcation angle decreases as expected. The behavior of mean angles for second order streams is reversed with angles of confluence decreasing and angles of bifurcation increasing. Evidently this property of drainage basins is not sufficiently sensitive to be significant in a sample of this size. Paraphrasing Horton, Schumm enunciated another law of drainage composition dealing with area: “The mean drainage basin area of streams of each order tends to approximate closely a direct geometric series in which the first term is the mean of first order basins” .7 Assuming, as suggested by LaForge and apparently substantiated by linear...

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