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Drainage anomalies over structural uplifts in regions of low relief (less than about 150 ft local relief) and unconsolidated rocks are of 2 types--drainage pattern anomalies and single-channel anomalies. Field studies and model investigations have revealed certain basic stream responses to local uplift. In areas of generally uniform regional slope, local reversals or marked divergences of drainage pattern from the regional slope are structurally significant. Radial drainage patterns (full 360° radial patterns) are always anomalous. Concentric drainage is due to the exposure of beds of different resistance and may, or may not, be present. If the unconsolidated material is uniform or very poorly bedded, the local shifting or deflection of drainage is significant, but i well-bedded rocks it may be merely homoclinal shifting downdip. Marked local, opposed drainage deflections are apparently due to almost continuous uplift favoring continuous lateral shifting rather than accelerated incision.
In unconsolidated rocks, the most important single-channel response to slight structural uplift is the local channel width-depth ratio. Changes in this ratio may be seen on aerial photographs. Such changes, however, may also be due to changes in lithology or in tributary streams. Decreases in gradient (such as upstream from a structure) result in increased channel width-depth ratio and increases in gradient decrease width-depth ratio. Meander compression or local changes in sinuosity may be due to structure or lithologic difference. Color aerial photography may aid in distinguishing structural and lithologic causes of single-channel features.
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