High mountain streams--Effects of geology on channel characteristics and bed material
Authors: Miller John P. 
Published: March 2023

Recent quantitative studies of drainage basins and stream channels have yielded significant results that promise greatly to enlarge our knowledge of fluvial processes and erosional landforms. However, the tentative conclusions derived from these preliminary investigations have raised in turn a host of fascinating new problems, which are now receiving attention by increasing numbers of research workers. Several current projects involve collection and analysis of new data, with reexamination and rigorous testing of certain basic concepts of geomorphology the immediate objective.

In recent years, several different approaches to the study of drainage basins and stream channels have been used, and a considerable variety has been investigated. From the results of these recent investigations, there can be little doubt that lithology and geologic structure affect the properties of drainage basins and stream channels in ways which are complex but nonetheless detectable by quantitative methods. However, the geologic and hydraulic data collected thus far are not sufficient in either number of areal scope to enable one to state very precisely what cause-and-effect relationships exist, or the extent to which they may differ in various geologic and geographic environments. Efforts at present are concentrated on accumulation of data and on sorting out the several independent and dependent variables. This is a slow process because of the relatively small number of persons involved, the complex interrelationships of the variables, and the large number of different kinds of areas to be studied.

This report adds new data from a high mountain area characterized by geologic and geographic conditions markedly different from those in areas previously studied. Besides the effects of different bedrock lithologies, extreme relief, and vertical changes in climate, the additional factor of glaciation might be expected to affect channel properties and the characteristics of bed material in high mountain streams. Furthermore, it has long been supposed that mountain streams are not graded; that is, they are not in equilibrium, but instead are actively downcutting. This implies that they differ in some possibly measurable way from equilibrium or graded streams. Hence, a principal objective in this study of mountain streams was to test the range of validity of certain conclusions reached in earlier investigations. Briefly, the procedure involved collection and analysis of stream data obtained in the high mountains, and comparisons of the results with information of similar character obtained in other, mostly nonmountainous, areas.

Any attempt to segregate geologic controls of characteristics and lithologic composition of bed material requires detailed knowledge of the bedrock geology of the drainage basin. Because the author had recently spent several field seasons mapping the geology of a large area in the southern portion of the Sangre de Cristo Range, New Mexico, the choice of a site for stream studies was resolved by the purely practical consideration of expediency. However, one additional advantage of this area over the other possible choices is the fact that the arroyos described by Leopold and Miller (1956) are located in the Rio Grande Depression, at the foot of the range, and afford a basis for local comparison. The order of discussion will be as follows. A brief resume of local geography and geology, emphasizing aspects of the physical setting, which affect stream properties, will define the general framework of these studies. Next, field procedures will be described, and data obtained from measurements of channel dimensions, bed material, and other properties will be presented. The subsequent sections will describe the results of efforts of geology on stream characteristics. Finally, the problem of grade or equilibrium in mountain streams will be considered.