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Methods <br />Cross Sections Approach <br />Cross section methods are frequently used to determine minimal flows for <br />maintaining habitats required by members of the aquatic community. These methods <br />operate by determining a stage - discharge relationship, usually across a riffle, and using <br />that relationship to find a flow that provides adequate depths and velocities for fish <br />habitat maintenance. This study used a total of 220 cross sectional profiles spread over a <br />distance of 128 river miles. Sample sites were selected by using the eight strata (river <br />reaches) described by Miller et al. (19 82) for the Yampa River (Table 1). Four strata are <br />located below Cross Mountain and three above Cross Mountain. Since Strata 5 (Cross <br />Mountain Canyon) and Strata 7 Juniper Canyon) were short, they were not included in <br />the sampling design. Sampling sites within the two study strata were randomly selected <br />by dividing the strata into 0.5 mile segments, named clusters. Each cluster was numbered <br />and 9 clusters were randomly selected from Strata 1, 11 from Strata 2, 3 from Strata 3, 2 <br />from Strata 4, 12 from Strata 6 and 13 from Strata 8. <br />Cross sectional measurements were taken during the base flow period in August <br />and, September in 1996 and in September and October 1997. Only one stage - discharge <br />measurement was taken at each cross section. The concern was for habitat availability <br />during base flow period, when flow typically ranges from 100 to 300 cfs. Cross section <br />were done at flow of 600 cfs or less and when flows were in this range only one <br />calibration flow was considered necessary to accurately model the relationship. <br />Cross sections were between headpins set at or above the grassline on both sides <br />of the channel. The slope of the bank and water surface elevation was measured using a <br />standard surveying level. Depth and velocity measurements were taken at 25 to 30 points <br />along the profile. The first cross section was placed at the most suitable hydraulic control <br />8 <br />