PATTERNS OF ALLUVIAL RIVERS

Abstract

The pattern (planform) of a river can be considered at vastly different scales, depending upon both the size of the river and the part of the fluvial system that is under consideration (Figure 1). For example, in the broadest sense, river patterns comprise a drainage network (dendritic, parallel, trellis, etc; Figure lA). The type of pattern is of interest to geomorphologists and geologists who interpret geologic conditions from aerial photographs. At another scale a river reach (which in Figure lB is meandering) is of interest to the geomorphologist who is interested in what that pattern reveals about river history and behavior, and to the engineer who is charged with maintaining navigation and preventing major instability. When a single meander is examined (Figure 1 C), the hydraulics offtow, the sediment transport, and the potential for bank erosion are of concern. In addition, the sedimentologist is interested in the distribution of sediment within the bend, bed forms within the channel (Figure lD), and sedimentary structures (Figure IE), which also establish a component of roughness for the hydraulic engineer. Finally, the individual grains (Figure IF) provide geologic information on the sediment sources, the nature of sediment loads, and the feasibility of dredging for gravel. There is an interaction of hydrology, hydraulics, geology, and geomorphology at all scales, which emphasizes the point that the fluvial system as a whole cannot be ignored, even though only a component of the system is to be studied. In this review only the patterns or planforms of alluvial rivers are discussed, although it is apparent that the hydrologic and sediment yield characteristics of the drainage basin (Figure lA), as well as its geologic history, cannot be ignored in the explanation of the pattern of any river