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0313 '-- ment-transporting discharge, which will be called the effective discharge. The frequency of the effective discharge depends upon the relative signifi- cance of large discharges as well as the magnitude of fluid forces required to erode the stream channel. Due to climatic, geologic and physiographic factors, the frequency of the effective discharge will vary from one stream to another, as well as longitudinally along a given stream. The frequency of the effective discharge decreases as the occurrence of large floods increases, i.e. as the tail of curve B (Fig. 1) is extended to greater discharges.The effective discharge, also, will become less frequent as the magnitude of fluid forces needed to erode the stream-channel increase. Baker (1977) has shown that rare, cata- strophic floods may be the effective geomorphic events in streams which have a high proportion of very large discharges or channels relatively resistent to erosion. The second principle concerns the relation between effective geomorphic forces and land forms. Wolman and Miller (1960) noted that in many instances, land features were formed by relatively frequent geomorphic forces and not by rare catastrophic events. In particular, they suggested that meander length and stream-channel size were adjusted to effective discharge. The concepts of magnitude and frequency of geomorphic forces expressed by Wolman and Miller (1960) have gained wide acceptance. Field investiga- tions, however, have often been inconclusive. In particular, it has been diffi- cult to relate land forms to geomorphic forces of a given magnitude and fre- quency. This is due principally to the lack of adequate data on the magnitude and frequency of geomorphic forces. Streamflow is probably the most exten- sively studied geomorphic process; even so, the available data are commonly insufficient to resolve these questions for three reasons: streamflow records are seldom of sufficient length to represent the full range of actual discharges, especially those that rarely occur. Sediment concentrations are measured infre- quently and are limited to that part of the sedimentload suspended within the flow. Thus, it is rarely ~ossible to compute the effective discharge of a given reach of stream channel accurately. Furthermore, the gaging stations at which sediment data are available are frequently located in reaches of channel stabilized or controlled by natural or man-made features. Thus, even for in- stances in which the effective discharge may be determined, the geomorphic effect of it may be muted or nonexistent. This paper describes a study of effective discharges and their geomorphic significance in the Yampa River basin. Streamflow and sediment data were collected from gaging stations located in self-formed alluvial reaches of stream channel to make this analysis. Y AMP A RIVER BASIN The Yampa River basin is located in northwest Colorado and southwest Wyoming (Fig. 2). It is divided into two subbasins of about equal area; the