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3. BOTTOMLANDS HYDRAULICS AND HYDROLOGY General Nature of Floodplain-River Interaction The occurrence and properties of flooded bottomlands are largely a function of discharge magnitude and channel morphology. The interaction between the river and the bottomlands can be affected by construction of levees or other barriers. The duration of flooding is the period during which a flooded bottomland area maintains the connectivity with the river. Ponded water remaining in depressions after the recession of the flood is not considered in the flood duration analysis nor is water ponded by seepage. Sedimentation and erosion processes in the channel can affect the connectivity through the development and removal of sandbars, or sediment deposition in the channels connecting the bottomlands and the river. Sedimentation and erosion processes in the channel can also induce sediment deposition in the bottomland areas, and changes in channel morphology during the rising and falling limbs influence the flood hydrology and bank erosion. The duration of flooded bottomland connectivity also can be affected by sediment deposition. During the falling limb, the loss of connectivity between flooded bottomlands and the river can result in ponded water that, without drainage, will isolate fish. However, initial fish counts in the Bonanza Bridge, Stirrup, and Old Charlie Diked floodplain habitats which were restored before the 1997 runoff season and sampled during the 1997 runoff season, indicated that few native and no endangered fish remained in the bottomland after the bottomland lost its connectivity to the river (UDWR unpublished data). Stage/Discharge Relationships Green River During the 1997 runoff season, four sets of water surface elevations were taken at six cross sections on the SR site. Discharges ranged from 12,300 cfs to 20,400 cfs, bracketing the flooding discharges of • interest. The 1997 peak discharge in this reach was 24,900 cfs on June 5th. The tabulated sets of water surface elevations can be found in the technical report. Three sets of measurements were taken on the rising limb and one measurement was taken on the falling limb of the hydrograph. Two sets of measurements were taken at approximately 18,000 cfs, one on the rising limb and one on the falling limb. With the exception of one cross section, the measured water surfaces were all within 0.1' of each other for the rising and falling limb measurement. The measured water surface elevations can vary up to 0.2' across the channel due to wave action, super-elevation, or other measurement inaccuracies. Near bankfull discharge, each half-foot in stage corresponds to approximately 2000 cfs. A 0.2' error in stage measurement could result in roughly a 600 cfs error in predicted discharge. During the 1998 runoff season, two sets of additional stage measurements were taken at the SN/IM property and three sets were taken at the SL/VR property. One additional set of measurements was made at the SR property. For the SN/IM property, the measured stages were taken at flows of approximately 14,000 cfs and 11,000 cfs. For the SL/VR property, stages were measured at flows of 14,000, 11,000 cfs and 8,000 cfs. These measured water surface elevations were used to re-calibrate the HEC-RAS model. The model was then re-run to produce more accurate water surface elevation predictions. The SR stage measurements were taken at approximately 11,000 cfs. These data were used to improve the stage/discharge relationship at the SR-site. Corresponding discharge data from the Green River near Jensen gage were utilized with the measured water surface elevations to establish stage/discharge relationships. A rating curve equation was developed for each cross section. A power relationship was developed between the stage and corresponding discharge by least squares regression. _I 3-1