7789 - Laserfiche WebLink

Home Browse Search

V. Summary and Conclusions A. Spawning Bar. After several high runoff seasons in the Yampa and Green Rivers since 1983, the large vegetated bar referred to as the razorback spawning bar appears to be relatively stable in shape. Some bank erosion is occurring both on the bar banks and outside channel banks. The bank erosion may be due, in part, to the apparent channel incision that has been observed throughout this reach possibly in response to upstream flow regulation. Future channel incision may be limited by the cobble substrate around the bar. The 20,200 cfs peak discharge was insufficient to mobilize substantial portions of the cobble substrate. It is expected that the channels around the bar which are confined by the steep slopes of bedrock outcrop on each side of the river will be stable for the long term. It might be postulated that during high flow years such as 1993, the bed would be scoured of sand during the peak and would have some sand deposited on the bed later in the seasonal flow hydrograph. This is not what happened in 1993. Large quantities of sand were deposited during the peak flow as illustrated in Exhibit A sheet 1, cross section E-4. The cobble substrate in the right channel, identified as a historical razorback sucker spawning area was buried with sand during the peak to a depth of 2 ft. By mid-August this sand deposit had still not yet been removed by the recessional limb of the annual hydrograph. Sand deposition in this reach is a function of the available sand supply. The immediate upstream supply is the sand stored in the river channel in the Island and Rainbow Parks' reach upstream of Split Mountain. Sand from the Park reach may move into the Escalante reach during high flow. This probably occurred in 1993. Although the sand storage in the Escalante reach in 1993 appears to be significant as observed in the cross section plots, the changes in channel geometry did not significantly effect the HEC-2 water surface elevation predictions. There was total sand inundation of the cobble substrate in the right channel between cross sections E-4 and E-5 during the peak discharge. This sand inundation of the substrate should not be expected every year and during some high flow years, it would be reasonable to assume that the cobbles would be scoured relatively free of sand. The effect of this sand inundation of the cobble substrate on the razorback sucker spawning habitat is unknown at this point; however, repeat years of severe sand inundation of the cobbles would undoubtedly have a negative influence on the reproductivity of the razorback in this reach of river. 31