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<br />31.2--2 <br /> <br />, <br /> <br />WATER INFORMATION MANAGEMENT SYSTEM HANDBOOK <br /> <br />and, thus, the one least likely to be in error in most cases. <br />However, the procedure does not preclude the use of a different <br />return interval if there are reasonable and substantial grounds <br />for its selection. <br /> <br />8. Baseflow discharge plays a role in the maintenance of <br />channel equilibrium. <br /> <br />Baseflow discharge helps to prevent vegetation encroachment into <br />the active channel and transports sediment when energy is <br />sufficient for the available sediment size and supply. <br /> <br />9. The natural concentration and yield of sediment of a <br />channel are dynamic properties and fluctuate about mean values. <br /> <br />10. The maintenance of existing channel capacity requires <br />the "run of the river" or all naturally occurring discharges of <br />a water and sediment mixture. <br /> <br />11. Approximate channel equilibrium can be achieved with <br />less than the "run of the river" through a range and duration of <br />flows including rise, bankfull, recession, and baseflow <br />components. <br /> <br />In view of assumptions 3 and 7, and as a practical matter, some <br />change to channel form from artificial activities must be <br />expected. However, the maintenance flows calculated in this <br />procedure are intended to minimize the adverse effects of <br />artlficial modifications of the normal flow regime. Further- <br />more, the calculated maintenance flow will result in sediment <br />transport at a level which lies within the range of expected <br />fluctuation about a mean value (assumption 7). This assumption <br />appears to be congruent with the intent of both Congress, as <br />defined in the Organic Administration Act [Act of June 4, 1897 <br />(16 U.S.C. 475)], and the Supreme Court, as expressed in United <br />States v. New Mexico [98 S. Ct. 3012, 8 ELR 20564 (July 3, <br />1978)]. <br /> <br />12. National Forest maintenance flow requirements will be <br />no more than those determined to be necessary for the <br />achievement of the objective of securing a favorable condition <br />of water flows. <br /> <br />13. Maintenance flows in ungaged watersheds can be <br />determined through methods other than direct discharge <br />measurement. <br /> <br />14. Gaged station flow-duration curves can be extrapolated <br />to nearby, similar, ungaged watersheds. <br /> <br />*-FSH 4/89 AMEND 3-* <br />