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The hydraulic factors of width, depth, velocity, and suspendedsediment load of ephemeral streams near Santa Fe, N. Mex., were measured during flood flow. Later, channel slope was measured. These flood-flow data, in conjunction with an analysis of drainage-basin configuration by the methods proposed by Horton, are used to determine the generalized interrelation of stream order and hydraulic variables. The method developed for determining this interrelation allows an integration of the geographic and physiographic characteristics of a drainage basin with the channel characteristics; specifically, the interrelation of the length, number, and drainage area of streams of various sizes with their respective discharge, width, depth, velocity, slope, channel roughness, and suspended-sediment load. These interrelations show that stream order is related to stream length, number of streams, drainage area, and discharge by simple exponential functions. The relation of discharge to width, depth, velocity, slope, and other hydraulic factors can be approximated by simple power functions. Thus, any pair of these factors is related by exponential or power functions. The data indicate that suspended-load measurements made during various stages of a few individual floods provide a close approximation to the suspended-load rating curve obtained from periodic measurements taken at a sediment station over a period of years. The analysis of the hydraulic data shows that in the ephemeral streams studied, velocity increases downstream at a faster rate than in perennial rivers. This appears to be associated with an increase in suspended-load concentration downstream in these ephemeral channels. The tendency for stream channels to maintain a quasiequilibrium with imposed discharge and load is shown to be characteristic of ephemeral channels in the headwaters of the drainage basin, even to the most headward rill. The formation of a discontinuous gully is analyzed in terms of this tendency. This type of gully is characterized by a gradient of the channel bed flatter than the gradient of the valley floor in which the gully is cut. The low gradient is explained as a hydraulic adaptation of channel slope to quasiequilibrium with the narrow gully width. Examples indicate that as a discontinuous gully widens, its gradient steepens and its length consequently increases. Thus a series of discontinuous gullies tends to coalesce into a continuous trench having a bed gradient nearly equal to the slope of the original valley floor. INTRODUCTION AND ACKNOWLEDGMENTS The flow of water in natural channels may be described as perennial, intermittent, or ephemeral. A perennial stream carries some flow at all times. An intermittent stream is one in which, at low flow, dry reaches alternate with flowing ones along the stream length. Those which carry water only during storms, and are therefore called ephemeral, are generally smaller but are much more numerous than perennial ones. From the divide to the mouth of a drainage basin, the increase in channel size is accompanied by a decrease in the number of channels. Drainage channels are more apparent and more abundant in arid than in humid regions where vegetation hides small rills. Vegetation also tends to increase the length of overland flow, thereby decreasing channel density. For both kinds of areas, however, the land presents to the eye a contrast depending on the perspective one assumes. In arid areas there is the panorama of broad, apparently smooth surfaces sweeping up to the abrupt fronts of mountain ranges or mesa escarpments. A closer examination may show that the whole countryside is actually cut into a myriad of rills, gullies, and arroyos. Humid regions, on the other hand, are typically characterized by rounded, rolling topography which is covered by natural or cultivated vegetation. Although many details of drainage nets are commonly concealed beneath vegetation, careful search discloses the presence of many more channels than can be inferred from a distant view. Despite the fact that the channels of ephemeral streams are generally recognized to have an important part in the erosion of the land and resultant production of fluvial landforms, they have not received careful or concentrated investigation.. The present report is devoted to a preliminary quantitative description of flow characteristics, channel properties, and configuration of the drainage net in arid regions only. However, PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS it is believed that the principles established may be applicable to humid regions as well. Practical considerations, along with personal bent, dictated that channels of arid rather than humid regions should receive our attention first. "Accelerated erosion" in the West is a problem of great social and economic importance. The arroyos that trenched alluvial valleys of the Southwest beginning in the latter part of the 19th century, some of which even now are eroding apace, are spectacular examples of ephemeral streams. Conservationists, range managers, geologists, and a host of others have debated the cause of gullying, have experimented with methods of gully control, and have tried to prognosticate the eventual status of the channel system. But in all such efforts practically no attempt has been made, to study the process of gully erosion itself, to describe the hydraulic conditions in the eroding channels, or to understand the nature of the equilibrium which was upset by grazing and climatic change. It seems bootless to spend large sums on the control of gullies and arroyos without concomitant attempts to increase knowledge of their hydraulic characteristics. The present study provides new data that are pertinent to a fuller understanding of ephemeral streams. Specifically, geometric and hydraulic properties of channels are related to the drainage-net .configuration. The plan of presentation is as follows: First, the channels in the area studied are described and the methods of investigation outlined. Next, the measurements of hydraulic variables, including channel shape, discharge, and sediment load are presented. A method of integrating hydraulic characteristics with properties of the drainage net is shown to increase markedly the utility of available data. Finally, the role of equilibrium in ephemeral channels, as related to the accelerated erosion problem, is discussed. Many colleagues and friends gave us continuing advice and assistance in the study, and particular thanks are due M. Gordon Wolman, Thomas Maddock, Jr., John T. Hack, and Walter B. Langbein. The field work was made considerably easier in many ways by the cooperation of Paul C. Benedict and Berkeley Johnson. Charles E. Stearns visited us in the field and later discussed the manuscript with us. We are indebted to John T. Hack and Stanley Schumm for permission to include in this report some of their unpublished data. GEOGRAPHIC SETTING AND BASIC MEASUREMENTS Nearly all the data included in this report were collected in the basins of the Rio Galisteo and Rio Santa Fe, tributaries to the Rio Grande in semiarid central New Mexico (see fig. 1).

Ephemeral streams - Hydraulic factors and their relation to the drainage net