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':'- 00 .-t o :--::.l ~'; Chapter III AFFECTED ENVIRONMENT AND ENVIRONMENTAL CONSEQUENCES The various agency personnel who worked on Stage Two investigations developed a technique for evaluating wildlife habitat based on values assigned to specific vegetation associations found in the Gcand Valley (Fish and Wildlife Service, 1984). Although several plant communities were considered, the associations, or "cover types" of potential importance to this study include marsh/riparian, tamarisk, saltbush, shrubland, and agriculture/edge. The agriculture/edge, and marsh cover types within the study area have been previously addressed in stage Two studies (Bureau of Reclamation, 1986a). Riparian vegetation is represented by small stands of cottonwood or willow, and shrubland consists of black greasewood with associated forbs. within ditch rights-af-way, most of the remaining habitat represented by these types is located along the Stubb Ditch. Impacts to existing wildlife habitat would involve loss of individual cottonwood trees and small areas of other woody vegetation. There are approximately 30 individual mature cottonwood trees growing along unlined portions of the Price Ditch, and approximately 35 such trees growing along the Stubb Ditch. On a worse case basis, it can be assumed that a majority of these trees would be lost during construction or die within several years after their water supply is removed. Irrigated lands and on-farm ditches immediately adjacent to the ditches may provide some moisture to the plants. In addition, the small berm and ditches associated with the O&M and maintenance road along the Stubb Ditch would trap moisture to preserve some water supply for the trees. Construction specifications would be written, however, to preserve as many of these trees as possible. Because of the scarcity of existing habitat along the Price Ditch, no other losses are anticipated. In addition to the loss of individual cottonwood trees along the Stubb DitCh, other cover types may be impacted. The portion of the Stubb Ditch immediately west of the Clifton interchange passes through an area supporting willow, cottonwood, tamarisk, and rabbitbrush. When this segment is realigned, approximately 1 acre of willows, 1.5 acres of cottonwoods, 0.5 acre of tamarisk, and 2 acres of rabbitbrush would be impacted by construction or reduction of water supply. The supply of water presently consists of seepage from the Government Highline Canal, precipitation collected by a berm along the Government Highline Canal, and seepage from the Stubb Ditch. East of this area, a total of 1 to 2 acres of greasewood may be impacted along the length of the ditch rights-of-way. No population estimates for species using these sites as habitat exist. However, the respective values assigned to each type during Stage One and Stage Two studies are assumed to represent habitat suitability as defined by the diversity and densities of small mammals and birds recorded from specific sample sites within the Grand Valley (Colorado Division of Wildlife, 1984). Using the formula: habitat suitability index (HSI) = average density-diversity divided by maximum density-diversity observed, cover types were assigned the following values: marsh/riparian, 0.76; tamarisk, 0.62; saltbush, 0.49; shrubland, 0.71; and agricultural land, 0.50. These numbers represent the value of each type of habitat to wildlife. The habitat evaluation procedures (HEP) (Fish and Wildlife Service, 1984) suggest that estimates of habitat suitability be multiplied by the area impacted to obtain a representation of future impacts. This representation, or habitat units (HUs) can then be used to compare impacts. For example, if individual cottonwood trees are assumed to influence approximately 4 acres, and this estimate is added to the 1.5 acres of cottonwood and 1 acre of willows along the stubb Ditch, then this 6.5 acres 24