2013-07-11_REPORT - M1988044 (2)

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Of greater concern is the amount of sand moving downstream. It has now almost reached the road crossing. This deep layer of sand influx is a serious threat to some of the stream channel vegetation. Because of its depth, wetland species, both herbaceous and woody, have difficulty growing up through the layer of sand to allow for some stabilization and the slowing of the movement of the sand downstream. The following two pages of photographs demonstrate where the sand is coming from. These were taken about 1/3 mile upstream from the road crossing and from the high east bank at the first sharp turn. The top photo on each page shows how the channel looked in August of 2002. At this time, intensive grazing was being done and it was near the end of a summer that began an intense drought that, eleven years later, is still going on with little more than a few breaks in the drought during that time. The lower photo shows the channel as it is today. In 2002, the vegetation, although generally continuous, was weak and composed mostly of grasses. By 2013 grass, sedges, willows, and a few cottonwoods had taken over, but there was also a considerable amount of bare sand that was not there in 2002. In 2002 there was a small headward erosion wall where noted in the photograph. Over the last 11 years this has crept upstream and is now cutting into the broad grassy area. The broad grassy area in the upstream photo set is a deposition area where the sand is deep and has even partially buried the bases of some of the larger cottonwoods. Now that headward erosion has cut into this deposition area, the sand deposited there is being released and carried downstream with each heavy flow. These headward erosion steps are common on prairie streams that are both intermittent and perennial. They even occur in large rivers like the Mississippi, Missouri, and Ohio rivers as well as the Colorado and Rio Grande. This is the normal way that streams that are degradational deepen their channel and lower the stream profile. It is done step by step rather than as continuous process with a gradual and equal lowering everywhere along the stream. Several of these steps can be found all along Coal Creek. Some are related to sand removal by mining, but most are not associated with mining influences and origins. The headward erosion areas can be found on the main channel as well as on tributaries throughout the Coal Creek watershed, even far upstream. That is because the rate of erosion is exceeding the rate of deposition and therefore the channel is descending in elevation, ultimately hoping to reach the base elevation of all streams and rivers - sea level. Obviously, that will take awhile, but it will likely happen - eventually. However, it is also true that although headward erosion removes material from one place it also deposits it elsewhere downstream and therefore, at those locations, the channel is temporarily aggradational and the local elevation is rising. With each flow, though, those deposition areas are moved down stream and spread thinner and thinner the greater the distance. But other degradational steps are moving upstream, so the ultimate effect is a sort of leap frog redistribution of stream bed materials down the stream channel to the sea. It was immediately wondered, upon seeing this headward erosion, whether it was due to the mining a short ways further downstream. Although there was initially a small headward erosion step as a result of mining and its removal of materials, that has now largely vanished as a result of erosion from upstream sources and deposition into the depression in the stream profile. In the downstream photo set it is important to note that other than some minor deposition of sand in the distant channel, that Status report for 2013 due July 15, 2013 Page 14 of 16