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Final Mitigation Plan for Long Hollow Reservoir <br />La Plata River Mitigation Site (Permit No. 200375243) <br />It should be noted that three piezometers also were installed in the area west of the pond. This <br />area appeared to be suitable for creation of wetlands, being relatively flat and downgradient of <br />the pond. However, the pits dug to install the piezometers found relatively dense, fine sediment <br />with cobbles, and groundwater was not encountered within five feet from the surface. This part <br />of the site was used for spoil reject material from mine operations. Therefore, it was concluded <br />that subsurface conditions have altered natural groundwater flow patterns and are not suitable <br />for wetlands creation. <br />The pond is fed by groundwater associated with the La Plata River. The water level in the pond <br />rises in the spring and early summer in response to snowmelt runoff in the river, and correlates <br />well with groundwater levels in the four piezometers in the mitigation area (see Table 2 and <br />Figure 5). There is a 24-inch culvert and an 8-inch PVC outlet in the southwest corner of the <br />pond which controls the water level of the pond. The water level of the pond decreases 1 to 1.5 <br />feet in the fall and winter. A staff gage was installed in the pond in December 2007. <br />Measurements to date for the gage are shown in Table 2. The staff gage will continue to be <br />measured when the piezometers are monitored. Water level information was used to design the <br />mitigation wetlands. <br />According to FEMA floodplain data on the La Plata River, the site is partially located within the <br />100-year floodplain of the La Plata River. The hydrograph for the river is typical of streams in <br />western Colorado: snowmelt runoff causes relatively high flows during spring and early summer, <br />and flows recede to baseflow levels the remainder of the year (Figure 6). In addition, diversions <br />for irrigation reduce flows during summer months. Typical spring runoff flows are 100 to 250 <br />cubic feet per second (cfs), and typical late summer flows are 5 to 10 cfs, based on the stream <br />gage at Hesperus. Flows are lower at the State Line gage. The reach of the river in the vicinity <br />of the mitigation site is reportedly spring fed and never goes dry, even during the extreme <br />drought of 2002 (personal communication, Mr. Pat Greer, long-time owner of the site). <br />Vegetation in the proposed wetland creation area consists of fair to good cover of upland <br />species, including western wheatgrass (Pascopyrum smithi4, mountain brome (Bromus <br />marginata), side-oats grama (Bouteioua curtipendu/a), and slender wheatgrass (E/ymus <br />trachycau/us). Some wetland species including Baltic rush (Juncus ba/ticus), fox tail barley <br />(Hordeum jubatum), and spikerush (Eieocharis spp) have invaded lower areas closer to the <br />pond. The site is relatively free of weeds, though several young tamarisks occur on the north <br />August 2010 Bikis Water Consultants, LLC Page 5