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328 \ agreement between the effective discharges and the bankfull discharges is ex- cellent. The standard euor is 14%. The effedive and the bankfull discharges are nearly equal for all the streams studied in this investigation. The close agreement shown in Fig, 11 appears to indicate that the stream channels studied are adjusted to their effective discharges, or more precisely, to the range of discharges that transport the vast majority of the sediment load of the stream over a period of years. The stream channels appear to be in quasi-equilibrium because floodplains have been constructed over a num- ber of years. Consequently, there has been a balance between channel erosion and deposition. That is, the stream channels were formed and maintained as much by deposition as by erosion. Because a vast majority of the annual sedi- ment load is transported by the range of discharges represented by the effec- tive discharge, the stream channels studied may be said to be formed by and adjusted to the effective discharge. Recurrence interval of the bankfull discharge li The recurrence interval of the bankfull discharge is of considerable practic- al. significance. Not only is it the flow of incipient flooding but also, as shown above, the bankfull discharge corresponds to the effective or channel-form- ing discharge. Therefore, it is useful to determine the bankfull discharge in order to consider many hydrologic (Emmett, 1975) and geomorphic (Dury, 1973) questions. Determining the magnitude of the bankfull discharge, how- ever, is not always possible and is, in fact, impossible when considering paleo- hydrology. These difficulties would be lessened if the bankfull discharge could be represented by a discharge with a constant-recurrence interval. Leopold et al. (1964) suggest that the bankfull discharge might be approximated by a discharge which has a recurrence interval of 1.5 yr. on an annual peak series. Dury (1973) used discharges with a 1.58-yr. recurrence interval. The recurrence interval of bankfull discharges in stream channels of the Yampa River basin are shown in Fig. 12. Note that the recurrence intervals are given for both the annual series (bottom scale) and the partial duration series (top scale)~ The Yampa River basin data are compared with the distribu- tion of 36 bankfull-discharge-recurrence intervals presented by Williams (1978). His data were collected entirely in the western U.S.A. from Texas to Alaska. - The two distributions are generally comparable, although the data present- ed by Williams (1978) included a significant number of observations with bankfull-recurrence intervals greater than 4 yr. The modal bankfull discharge in the Yampa River basin is between 1.18 and 1.40 yr., while that for Williams' data is about 1.5 yr. Both sets of observations, however, show that there is a large variance in the distribution of bankfull-discharge-recurrence intervals. Fifty percent of the streams in the Yampa River basin had bankfull discharge recurrence intervals greater than 1.75 yr. or less than 1.25 yr. Consequently, there does not appear to be a common frequency of occurrence for bankfull discharge. .~ ) \ ,