2009-08-01_HYDROLOGY - M1981112 - Laserfiche WebLink

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7 Figure 16 illustrates the project site five years later in 1998. The River has abandoned the north most braided channel on the left bank and has flooded the mining pond on the right bank. Specifically, in about 1997, 3-4 feet of overburden was removed from the area west of the mining pond so that mining could extend to the west property boundary. During a period of high water, the River flooded this area all the way from the west boundary through the mining pond and to the River again. A dam was created along the west property boundary and a smaller one was created on the east end of the flooded area. The area of wetland along the River on the east end of the right bank is likely quite small. Figure 17 illustrates the project site five years later in 2005. Due to the flooding, the mining pond has been moved back to the north and the area of the old mining pond to the west is dominated by a cattail marsh. The River continues to rework the sand on the left bank, and a Herbaceous Wetland has developed on the right bank of the River north of the 1970's spoil pile, partially in the area lowered by the flood that flowed through the mining pond. Please note, the entire area of the Herbaceous Wetalnd was initially excavated in the 1970's. Then following the flood, a large portion of the Herbaceous Wetland site was again excavated for masonary sand because when the River flooded the mining pond, mining operations were shut down for a period of time, and therefore the masonary sand was excavated and sold. The mining pond is drained by a small channel that meanders through the wetland, and a berm has been created along the west edge of the wetland. Figure 18 illustrates the project site three years later in 2006 before the excavation on the left bank and before fill was placed in part of the Herbaceous Wetland on the right bank. The Herbaceous Wetland abuts a berm to the west, which was created per the requirement of the Colorado Division of Reclamation, Mining & Safety to keep the River water out of the mining pond. Some time after 2006, Builders Aggregate Company moved the sandy berm into the Herbaceous Wetland on the right bank in order to keep sand from washing into the River. Figure 5 illustrates the area of fill, which is about 10,464 square feet. Photo 13 illustrates the unfilled portion of the Herbaceous Wetland and Photo 14 illustrates the berm that was placed in the wetland. Please note, the area west of the berm was once part of the wetland. Photo 15 illustrates an area of the right bank that was used by Builders Aggregate Company as a Low Water Crossing (Figure 5). This site has no fill and it was not excavated, although the sand is slightly compressed due to the weight of vehicles moving through the site. The only impact here was the removal of wetland vegetation over an area of 57 square feet. 3.0 Proposed Physical Conditions 3.1 Proposed Remedial Work – Left Bank 3.1.1 Sandbar The development of the sandbar is a normal process expected at this location on the River. The aerial photos (Figures 11-18) illustrate that the River has been forming a single thread bankfull channel in recent years and has been filling in the remnant overflow channel to the north. Neither excavation nor fill in this area will benefit the River channel. The proposed treatment for this sandbar is no action. The 2009 runoff was quite high and deposited material along with a seed bank (Photo 11). Unless the flow in 2010 is higher and deposits even more material, the seed bank in the deposited sand and gravel will produce a population of sandbar willows in subsequent years. 3.1.2 High Flow Channel Approximately sixty cubic yards of fill was placed in the High Flow Channel when Builders Aggregate Company created a road across the Channel (Photo 12). This material will be removed to restore the 351 square feet of impact and the approximate original grade of the Channel. Figure