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<br />~~r <br /> <br />'" <br /> <br />'_~:r-;;~J' . <br /> <br />001553 <br /> <br />. <br /> <br />. <br /> <br />APPLICATION OF THE FOREST SERVICE <br />CHA}1NEL MAINTENANCE FLOW PROCEDURE <br /> <br />, 11 <br />James D. Schall and Peter F. Lagasse- <br /> <br />DIR \.,' <br />--'-:'I. ., <br />SC ' '--- ". ..' <br />,~\",,'\'~L._:,~ ' <br />~5...'."'[~" -.~,'~-- ", ':' .... "...... <br />F' '.:' . ;: <br />\ \ <br />L __-:;:::~\ <br />! -;:::::=:.:;:~.- <br /> <br />INTRODUCTION <br /> <br />The U. S. Forest Service has recently adopted nationwide a channel <br />maintenance flow procedure (see Chapter 30 of the Water Information Manage- <br />ment System Handbook, FSH 2509.17). As described, the channel maintenance <br />flow procedure is intended to quantify ". . . those streamflows necessary <br />to provide for the self-maintenance of river, stream, or creek channels so <br />as to retain their capability for passing flood-flow discharges" (Section <br />30.2, FSH 2509.17). A basic concept in the procedure is that natural <br />channels are self-formed and self-maintained and have developed in a <br />predictable manner as a result of:upstream water and sediment loading. .Two <br />primary concerns addressed by the procedure are, (It ,maintaining 'sediillf!nt' <br />transport capacity', to prevent aggradation, and (2) minimizing yegetation'. <br />encroachment. <br />. <br /> <br />The Rock Creek Reservoir project is one of two elternatives being <br />considered as compensatory storage for the Windy Gap project. Rock Creek <br />is tributary to the Colorado River near McCoy, Colorado. Preparation of <br />the environmental impact statement for the Rock Creek project required <br />application of the channel maintenance flow procedure since the proposed <br />reservoir sites are located within the Routt National Forest boundary (see <br />Fig. 1). <br /> <br />.--~-=- <br /> <br />APPLICATION OF CHAPTER 30 to ROCK CREEK <br /> <br />The first step in the Chapter 30 procedure is determination of bank- <br />full discharge at the point of quantification. The point of quantification <br />is that location (or reach) where prevention of aggradation or vegetation <br />encroachment is judged to be most critical to maintaining channel capacity. <br />FClr the Rock 'Creek project,' two alternative'. reservoir sites are under <br />consideration. The upper site (Site A) is located just below Horse Creek' <br />while the lower site (Site B) is near the entrance of a steep canyon about' <br />4,000 feet below Site A. Based on the USFS stream classification proce- <br />dure, the reach below Site A is a C3 reach while the reach below Site B is <br />a Bl reach. A C3 reach is a relatively mi,ld sloped, gravel"bed channel of <br />moderately high sinuosity while a BI reach' is a steep, relatively straight <br />channel composed of large cobbles to small boulders. Fig. 2 is a plot of <br />the Rock Creek bed profile illustrating the differences in channel slope <br />downstream of the two alternative sites. <br /> <br />Evaluation of bankfull discharge was based on field observations and <br />D:easurements, data from the USGS gage' Rock Creek near Toponas (Station <br />09060500), and from previous analysis on Rock Creek completed by (8). <br />Results indicated that a reasonable bankfull discharge below both sites (A <br />and B) was approximately 190 cfs. <br /> <br />l/ Water Resource Engineer and President, respectively, Resource Consul- <br />tants, Inc., P. O. Box Q, Fort Collins, CO. <br /> <br />~-~ <br />