Controlling factors in the distribution and development of incised meanders in the central Colorado Plateau

A study of the distribution and geometry of incised meanders in 64 reaches encompassed approximately 600 km of the Green, Colorado, and San Juan Rivers in the central Colorado Plateau. The sinuosity, average planform size, and average cross-sectional symmetry of each reach were determined by map measurements and by spectral analysis of the curvature series for each reach, as determined from interpolations of the digitized traces of the channels. Possible controlling variables examined, including average channel gradient, drainage area, average bedrock erodibility, and bedrock structure, were compiled for each reach, using available maps. Gradients in the studied reaches are significantly correlated with bedrock type.Sinuous incised channels are generally found in low-gradient reaches. In the San Juan River, the channel is sinuous where it flows against the bedrock dip and generally straight in reaches where flow is downdip. This correlation is weak in the Green and Colorado Rivers. The average meander size of the sinuous reaches, as described by the median curvature value for each reach, is generally less in steeper reaches than in low-gradient reaches, although the relation of bend size to controlling variables is much less clear than for sinuosity or cross-section shape. Most meander cross sections in the area are relatively symmetrical, but highly ingrown forms are also present. In general, symmetric bends are associated with resistant bedrock units, whereas ingrown forms develop in massive sandstone and in highly erodible bedrock. Gradient significantly influences the distribution of ingrown bends, with asymmetric meanders concentrated in reaches of low average gradient; this correlation is stronger than that between cross-section shape and lithology itself.Incised meanders of the central Colorado Plateau are probably at least partly inherited from ancestral streams of unknown age that flowed across the area before the present canyons were cut. Correlation between meander distribution and regional structures suggests that the general location of low-gradient sinuous reaches has probably not changed during the incision of the present canyons. Incised meanders, however, are clearly able to modify their geometry in response to changes in bedrock resistance, as indicated by the strong correlation between bedrock type and cross-section symmetry. One mechanism of modification is abandonment of bends, which is documented by 18 cutoff meanders in the study area.