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Memo to Dan Mathews <br />Observations re: Powderhorn Coal Field Visit <br />page 3 <br />aggradation within the relocated channel. The channel became choked with sediment and has <br />attempted to break out and bypass several of the check dams. In addition the check dams are also <br />being undercut on their down gradient side and have failed in places, requiring reconstruction of <br />the effected check dams. Finally, aggradation has progressed up stream to a culvert & concrete <br />lower water structure, causing this structure to plug with sediment. Concern is now focused <br />upon maintenance of the channel, both in the immediate and the long-term scenarios. <br />In hind sight, once again, what has happened should have been anticipated. Coal Canyon Creek <br />represents a typical book-cliffs, badlands, arroyo regime stream. When the occasional high flow <br />event occurs the stream mimics a granular mudflow. The combination of precipitous slopes and <br />poor soil cover results in extremely steep hydrographs. The high velocity flow mobilizes the <br />granular soil. resulting in sediment saturation of the stream. Flow in these water sheds tends to <br />be highly erosive in the upper most reaches of the water sheds and depositional in the lower <br />reaches. Significant erosion in the lower reaches is largely limited to the beginning of rising limb <br />of the hydrograph and the tail portion of the falling limb. All other flows are saturated and <br />therefore probably relatively nonerosive. The saturated condition results in an enormous amount <br />of sediment passing through the channel and into the Colorado River. The main impact of the <br />Powderhorn Coal check dams appears to have the placement of tens of thousands of cubic yards <br />of sediment into storage within the relocated reach of the Coal Canyon channel. <br />Without dwelling on the fascinating geomorphology of the situation any longer, I will present my <br />recommendations: <br />(1) Harvest the accumulated sediment for use as subsoil cover for the as yet unreclaimed <br />upper benches and tops of RDA#1 and RDA#2. The approved plan called for harvesting <br />subsoil from adjacent undisturbed areas of the permit. Removal of this material from the <br />channel will prevent its immediate migration downstream into the Colorado River. <br />Although, I suspect it's migration into the Colorado would represent a relatively <br />insignificant contribution to both the suspended load and bed load of the Colorado River. <br />(2) Remove the check dams. I suspect the oversize material comprising the dams could serve <br />a valuable purpose as armor or a deflector berm for the adjacent toe of RDA#l. The toe <br />of RDA#2 appears to be sufficiently elevated to be out of threat from Coal Canyon <br />channel migration. <br />(3) Remove the culvert and concrete ramp crossing above the relocated reach of the channel. <br />This will allow the channel gradient [o adjust as necessary to accommodate the relocated <br />channel. <br />