Dolores River Core Science Report

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..:° a,. ~~ ~. w ,. ~.~,: '~~ ~.~ - -'.. - ;~ _ «. ~" _ `° ~ ~~. a ~lgi ~ - d _ r r .~~y~ ' ~ 6 ~ ~ ~~~ Photograph of Dolores Rig er in Lone Dome State Wildlife Area beri~ een McPhee Darn and Bradfield Bridge. shoeing reach that has been nai7o"ed tluongh importation of cobble as part of CDOW 1~3bitat impro~-ement project (from CDOW, 2003). Bradfield Bridge to San Miguel River Downstream of Bradfield Bridge, the river steepens and enters the Dolores Canyon, a deep carryon (up to 2300-ft. deep) with steep-sloped or stair-stepped canyon walls (CO DNR et al., 1976). The reach from Bradfield Bridge to Disappointment Creels (a 41-mile reach ending several miles upstream of the town of Sliclcrock) is the steepest reach downstream of McPhee Dam, with an average gradient of 0.45% (24 ft/mile) (CO DNR et al., 1976). Marry segments of the river in Dolores Carryon are likely bedrock-controlled. Although the river has some sinuosity as it follows the entrenched meanders of the canyon walls, channel migration is limited due to the narrow valley bottom. A median grain size (D50) of 222 mm (large cobble) was measured near the Dove Creek pumping station in the late 1980s (USDI BLM, 1990). Below this location, a dirt road parallels the river for about 11 mile. Tlus reach likely has a high sediment transport capacity because of its confinement and steep gradient. Downstream of Disappointment Creek, the river alternates between confined canyons and wide, structural valleys (CO DNR et al., 1976). Tlus portion of the Dolores River is characterized by both reduced gradients and increased fine sediment loads (CO DNR et al., 1976), resulting in finer bed material than upstream reaches. For example, a median grain size of 60 mm (large gravel) was recorded near the confluence of Disappointment Creek in the late 1980s (USDI BLM, 1990); more recent data are not available. Sediment inputs from the Disappointment Creek drainage are particularly high and are discussed further below in the section on sediment dynamics. Below Disappointment Creek, after a short (6 mi.) canyon section, the Dolores meanders with a low gradient through Big Gypsum Valley, a broad valley that contributes minimal nuioff to the mainstem. Bed substrates consist predominantly of siltsize materials, although the presence of coarser sediment along exposed bars suggests that fiiung may have occurred in this reach due to reduced flows and increased fine sediment supplies. From Big Gypsum Valley to Bedrock (a short distance upstream of the San Miguel River confluence), the river flows through Slickrock Carryon, a narrow canyon of highly sinuous, entrenched meanders flanked by steep sandstone cliffs, some 21