Dolores River Core Science Report

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Dolores River downstream from Bradfield Bridge was in a "near natural" condition, being dominated by sandbar willow and extensive groves of cottonwood in the late 1970s. At the time, tamarisk was abundant downstream of Disappointment Creek and became increasingly dominant downstream (USDI 1979). The report indicated that there were extensive groves of cottonwood in the Gypsum Valley in the late 1970s. There are only a few old cottonwood in the valley today, but tamarisk is abundant, and monotypic stands are extensive (photos and field visits). Narrow canyon reaches of the Dolores River are not well-suited to support extensive cottonwood forests and are more likely to support shrub and dwarf tree commuiities (birch, sandbar willow, box elder, etc.). Wider valley segments are more conducive to the formation of extensive cottonwood forests. Tamarisk was abundant along the Dolores River downstream of Disappointment Creek by the late 1970s but was and remains less dominant upstream to McPhee reservoir. Tamarisk is limited by frost and is thought to become abundant only below an elevation of about 7,000 ft on the West Slope in Colorado. It is not surprising then that tamarisk is not dominant upstream of Bradfield Bridge, but becomes increasingly dominant in low lying alluvial reaches downstream such as in the Big Gypsum and Paradox Valleys and near the confluence with the Colorado River in Utah. Because saline soils favor tamarisk, areas with high levels of salt may result in tamarisk being at an advantage over some native species along some reaches of the Dolores River. Sediment supplied to the river is exceptionally concentrated with salt due to exposed Marcos shale formations in tributaries and along the Dolores River valley. Salt has been further concentrated in low lying areas due to evaporation of water from shallow water tables and subsequent concentration of salt in the upper soil horizons. The visible salt crusts in some areas are lalown to ii~libit the gernunation of cottonwood seeds and the survival and growth of cottonwood seedlings (Shafroth et al. 1995). Occasional overbank flooding could serve to flush floodplain soils and improve conditions for cottonwood and willow establishment and maintenance. Highly saline soils can increase the vulnerability of poorly adapted plants to problems associated with moisture stress. Kriegshauser and Sommers (2004) measured riparian vegetation along a reach of the Dolores River near Lone Dome from 1988 through 2001 and recorded long-term trends. They recorded a significant increase in sandbar willow cover and number, no significant change in narrowleaf cottonwood cover or number, and a decline in silverberry over the 14 years considered. They noted that streamside meadows appeared to have become dominated by xeric (dry adapted) species. They noted that meadows historically flooded at 8,000 cfs or more, had rarely been flooded since the installation of McPhee Dam in 1984. The increase in sandbar willow noted by Kriegshauser and Sommers (2004) is not surprising as sandbar willow can spread vegetatively by root sprouts so are not as dependent on hydrograph attributes for asexual reproduction. Since peak flows have been reduced on the Dolores (by 48% at Bedrock, CO; 20% at Cisco, UT), populations of willow that might have been kept from encroaching on the channel by scouring annual flows have slowly colonized the formerly active channel. The encroachment of willow is a predictable response to the reduction in annual peak flow. Particularly along gaining reaches or reaches with stable baseflow. Sandbar willow provides valuable habitat and is 50