Morphometry and floods in small drainage basins subject to diverse hydrogeomorphic controls

Morphometric parameters such as drainage density, stream magnitude, and relief ratio are practical measures of flood potential in small (<100 mi2) drainage basins. Stereoscopic interpretation of low-altitude aerial photographs provides the most accurate maps of basins for generating these parameters. Field surveys of a high-density limestone basin in central Texas show that 1:24,000 scale topographic maps accurately portray the efficient stream channel system but fail to reveal numerous small gullies that may form portions of hillslope hydrologic systems. Flood potential in drainage basins can be defined by a regional index computed as the standard deviations of the logarithms of the annual maximum streamflows. High potential basins tend toward greater relief, greater drainage density, and thus greater ruggedness numbers than low–flash flood potential watersheds. For a given number of first-order channels (basin magnitude), flash flood regions have greater ruggedness numbers, indicating higher drainage densities combined with steep hillslopes and stream channel gradients. Transient controls on flood response, such as differences between local rainstorm intensities, appear to be the major influences on hydrographs in areas of moderate dissection and relief. Morphometric parameters for low-potential flash flood regions )Indiana and the Appalachian Plateau( are better estimators of frequent low-magnitude runoff events (mean annual flood), while the same parameters correlate better with the maximum flood of record in high-flood potential regions (central Texas, southern California, and north central Utah).