Dolores River Core Science Report

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a desirable species along western streams, but encroachment on the chamiel has the negative effect of reducing channel capacity. Reductions in the cover and number of silverberry could be related to increases in soil salinity associated with reductions in the magnitudes of peals flows. Silverberiy is tolerant of occasional flooding and can resprout after fire or scouring floods. Silverberry is often located in areas that may be considered transitional between the lower more frequently flooded areas where sandbar willow occurs and higher drier areas dominated by big sage and Pinyon-Juiuper. The Colorado Natural Heritage Program inventoried seven sites along the mainstem of the Dolores River from McPhee reservoir to downstream of the confluence of the San Miguel River in the late 1990s. They identified more than 50 plant species, and seven different plant associations. Three of these associations are considered to be globally imperiled due to rarity. These associations include box elder/river birch, box elder-cottonwood/red-osier dogwood, and strapleaf willow. Other associations documented included three square bulnish, sandbar willow/mesic graminoid, and slcunkbrush. Associations along tributaries of the Dolores River included spiken~sh, box eder/river birch, and silverberry. Although these samples are not likely representative, cottonwood was not dominant in any of the stands sampled on the mainstem of the Dolores River, whereas it was the donunant in six of seven sites sampled on the lower San Miguel River within 10-15 lan of the confluence with the Dolores River. The San Miguel is undammed and is considered to be one of the few relatively free-flowing rivers in the Colorado River basin. The plant associations identified above could be used to identify species to manage for and assist in the establishment of management goals targeted to specific riparian species or communities. Because these reports and studies occurred over several decades and along different segments of the Dolores River, it is difficult to draw an overall assessment of the current state of the riparian vegetation of the Dolores River from existing literature. It would appear that cottonwood forests are less frequent and less extensive along the Dolores River than they were historically, that silverberry may be declining along some reaches, and that tamarisk has increased in abundance and extent (as it has on many western rivers including the free-flowing San Miguel) and that sandbar willow is more abundant than it was historically. It is likely that perennial species are more abundant along the Dolores River as well and that obligate wetland species have increased in abundance since relative stabilization of flows in the decades following the construction of McPhee dam (Merritt and Cooper 2000). Given this cursory assessment drawn from scant existing data, it becomes clear that a vegetation inventory of the Dolores River would provide invaluable baseline data on the current condition of the vegetation and would facilitate the tracking of trends in the condition of the vegetation through time. An assessment of current condition of riparian vegetation along the Dolores River would assist in setting goals for management and shed light on the processes that could be most efficiently manipulated to achieve some desired outcome. An inventory would also provide valuable information that could be used for setting realistic goals that account for management and physical constraints. It is likely that the current stricture of the riparian vegetation along the Dolores River is an integration of the changes that have occurred not only in flow regime, but also in grazing, human use, and other factors. 51