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Awgwtt 1992 Gyprxt TGC • Swlaidrncr Efferu on Vegrmtion ~ 3R <br />• total vegetation cover and big bluegrass accounu for approxinutely 24 percent of the total. <br />Mean total production in this type (based on 1984 data) wu 149 grams per square meter. Shrubs <br />acrnunted for 59 percent of the [oral production and puennial grasses accounted for 25 percent. <br />The sagebrush/meadow type on the bottom-land hu a mean cover value that is 18 percent <br />greater and a production value that is approzimately 98 grams pu square meter higher than the <br />sagebrush shrubland type that occurs on the adjacent uplands. <br />2.3.3 Fleld Data <br />During June and July of 1992, a vegetation study wu conducted for portions of the Fish Creek <br />floodplain within the study arc. Cover and production data wee obtained from two site area <br />within the valley bottom. Site A is located within the area potentialy impacted by longwall <br />mining and rela[ed subsidence, while Site B is looted dowttsveam and outside of the potentially <br />impacted area. Dan for these sites are rummarized by Tables 2 and 3 which are organized using <br />the same format utilized by Colorado Yampa Coal Company in reporting the results of previous <br />and ongoing vegetation monitoring. <br />3.0 SUBSIDENCE EFFECTS <br />In order for any rubsidence effects to oaur within the sagebrush/meadow vegetation type, the <br />subsidence would have to affect existing plant-soil-water relations for the existing vegetation <br />types. In theory, the rystem could be disrupted by either a decease in water availability, if the <br />• subsidence caused a dewatering of the alluvial rystem, or an increase in the amount of wuu <br />available to the plants if the rubsidence were to disrupt and impair exining flows and drainage <br />patterns. If either of these conditions wee [o ooatr on a long term bui; vegetation on the <br />bottom-land area could be effected. <br />In the event the alluvial rystem were to be dewatued, the vegetation would likely change and <br />become more like the adjacent upland sagebrush vegetation type. Total vegetative rover in the <br />bottom-land could decease by up to 20 percent and productivity could drop a muimum of 100 <br />grams per square meter. The magnitude of these changes is based on the assumption that oven <br />if the bottomland area are dewatered, vegetation on the valley floor could continue to product <br />u much biomes and provide u much cove u the adjaont upland vegetation. Information <br />from the subsidence (SubTura/ACZ 1992, F.:hibit 7B) and hydrology (Hydro-Geo <br />Consultants/ACZ 1992, Ezhibit 45) reports suggest that undumining the alluvial valley floor <br />will not dewatu the stream valley is the portions that are undermined. If the alluvial rystem <br />is no[ dewatered, thue will be no significant chutges in the ezisting vegetation. <br />If non-uniform subsidence resulting from undermining wee to a disrupt drainage pattuns, it <br />is pouible that vegetation on the valley floor could became more mesic, the pucentage of sedges <br />and rushes could increase, and the vegetation could change to more of a wetland meadow type. <br />If this wee to oavr, vegetation parameters including total vegetative cover and mean <br />production would likely increase over atrrent values. Information from the hydrology and <br />subsidence reports suggest chat subsidence could minimally influence the stream gradient, <br />howevu 'the dynamic character of Fish Creek, with a constantly changing rhattnel atting <br />through fine grained sediment; would fadlitace rapid natural adjustments to any temporary <br />. changes in streambed gradient' (Hydro-Gto Consulnnu/ACZ 1992). ff thue are no changes <br />in the stream gradient and the assodated alluvial deposit; thue would likely be no signifiant <br />changes in the ezisting vegetation. <br />'~°"'L' ACZ Ire. • P.O. Bos 774018 • Steamboat Springy, Colorado 80177 • (303J87%260 <br />crew y. <br />