Principles of underfit streams

Meandering valleys with systematic windings are defined as a subclass of winding valleys, the bends of which can be indifferently systematic or irregular. The terms "valley meander" and "valley bend" are correspondingly applied to single features, although they are conveniently interchangeable in practice. Free meanders on a flood plain are distinguished from incised meanders, which are subclassified into intrenched and ingrown forms. Misfit streams, either too large or too small for the valleys in which they flow, are grouped as overfit and underfit. Wavelength of meanders is used as a criterion of size and is statistically referable to discharge at the bankfull stage. The term "underfit stream" also is applied to streams that either fail to describe meanders in their present channels or that lack valley meanders. Meandering streams in more amply meandering valleys are called manifestly underfit. Stream capture or other forms of derangement of drainage are incapable of supplying the general hypothesis of the origin of underfitness that is required by the facts of distribution. A critical reexamination of W. M. Davis' attempts to explain underfitness by capture reveals grave weaknesses in his argument. Authentic diversions from the French Rivers Meuse and Bar were less effective in reducing wavelength of meanders than was the change which made all the neighboring rivers also underfit. The Wabash and Souris Rivers, the drainage on the emerged floors of glacial Lakes Agassiz and Harrison, and the back-slope streams of the English Cotswolds provide data on spillways. These data show that overspill from proglacial lakes, or direct discharge of melt water from ice fronts, is irrelevant to the general problem of underfitness as it is separable from reduction of stream volume in either space or time, or both. Regional development of manifestly underfit streams requires a climatic hypothesis of origin. Examples of such streams, from the till plains of Iowa, from the Humboldt, Shenandoah, Salt, and Cuivre Rivers, and from the Ozarks, show that lack of manifest underfitness can be due either to the destruction of relevant valley forms by erosion or to the failure of streams to develop meanders in their existing channels. Alternatively, this lack may be merely apparent, owing to deficiencies of map evidence. Where stream meanders are actually lacking, pooland-riffle sequences can occur with spacing that is appropriate to stream meanders and far closer than that appropriate to valley meanders. Davis' claim that the terraces of the Westfield River indicate no significant change in volume during terracing is directed at the views of Emerson. But the claim seems not to bear on the general problem of underfitness, especially as manifestly underfit streams occur on the emerged floor of glacial Lake Hitchcock. In Arizona, Padre and Diablo Canyons and Oraibi Wash are manifestly underfit streams, providing evidence in arid regions of reduced channel-forming discharge comparable to that noted for humid regions. In association with observations for the Humboldt River, this evidence extends the general hypothesis of underfitness to regions that are currently semiarid or arid. Contact of streams with bedrock is provisionally rejected as a direct cause of the absence of meanders from present unbraided channels, and variation in cohesiveness of alluvium is suggested as an alternative. The view that incised meanders are necessarily inherited from free meanders is controverted. Numerous minor canyons are thought to act as flumes after heavy rain, but valley nets in dry regions are inferred to have been intitiated and developed under more humid conditions than now exist.