Laserfiche WebLink
<br />002455 <br /> <br />The widespread bed degradation has resulted in the emergence of additional <br />channel controls, which frequently occur at the mouths of tributary canyons. This is <br />evidenced by the development of a stepped low-water longitudinal profile from the <br />smooth profile that existed in the pre-dam era. The magnitude of degradation in riffles <br />decreases with distance downstream from the dam, consistent with general models of bed <br />degradation downstream from dams, The magnitude of sediment evacuation from pools, <br />however, does not decrease systematically, but fluctuates in magnitude and extends much <br />farther downstream. <br />The alluvial deposits in Glen Canyon include a complex suite of pre- and post- <br />dam fine- and coarse-grained deposits that occur at a variety of elevations above the <br />active river channel. Many deposits are pre-dam remnants, specifically associated with <br />the lowering of the channel. These include pre-dam fine-sediment deposits that are now <br />perched high above the present active channel and mid- and side-channel gravel and <br />cobble bars that occur in segments where degradation was concentrated in one part of the <br />channel, leaving exposed large areas of the pre-dam riverbed. The reach also includes <br />narrow strips of fine-grained post-dam flood deposits. Both pre- and post-dam fine- <br />grained deposits have been colonized and stabilized by invasive exotic riparian <br />vegetation, resulting in a net decrease in bankfull channel width of about 6% throughout <br />the study area. <br />Some erosion of channel-side deposits has occurred in the study reach, although <br />the magnitude of this erosion is small compared to the evacuation of sediment from the <br />bed. Most of this erosion has been concentrated in a few pre-dam terraces that eroded <br />dramatically for brief periods and have become armored and stabilized. The area of <br />alluvium represented by eddy deposits has not changed significantly, but the elevation of <br />these deposits has decreased resulting in net erosion from eddy storage environments. <br /> <br />5 <br />