EXTREME HIGH AND LOW DEGREES OF DRAINAGE DIVIDE WANDERING, CONTERMINOUS U.S. CONTINENTAL DIVIDE

The map-view forms of drainage divides are fairly consistent in their degree of wandering, with fractal dimension (D) values typically in the range 1.06-1.12 as quantified by the divider method. This has been documented in studies of drainage basin boundaries in humid-temperate, tropical, and arid regions, and in a physiographic-province-scale study of the conterminous U.S. Continental Divide. However, a geometric analysis of localized segments of the Continental Divide, with resolution range 100’s m – 10’s km, gives some examples of extremely high trace wandering (D values 1.14-1.16) and extremely low wandering, or near-smooth form (D values 1.01-1.04).

Of 65 segments comprising the full Divide length, twelve have extreme D values. These cases are confined to three clusters along the Divide trace: six segments in southern New Mexico, two the northern Colorado Rockies, and four in the greater Yellowstone region. Each of these three clusters includes cases of both high and low extreme values. Overall bedrock geology appears to make a difference in occurrence of extreme values. Half of the extreme cases are in areas of Quaternary and Tertiary extrusives; none are in areas with exclusively sedimentary bedrock.

Five segments display extremely low values of divide trace wandering. In all cases but one, the near-smooth geometric character is observed only within a relatively coarse resolution range (approx. 4 to 35 km divider step size), with more typical values of wandering, D ³ 1.06, found at finer scales (approx. 0.4-2.5 km). These five segments are notable for divide paths matching a distinctly linear structural control, flanking a nearby major drainageway, or running between stream courses of a parallel drainage pattern on mountain piedmont.

In all but one of the seven cases with extremely strong divide wandering, a single, high D value characterizes segment geometry at fine through coarse resolutions (approx scale range 1.7-15 km). These segments traverse areas with strong disruption of drainage by volcanic, glacial, and structural influences, and with differing degrees of dissection among competing watersheds. In some of these extreme cases, the divide path connects a series of isolated mountain uplands or buttes with dominantly radial drainage patterns.