Mancos Watershed Functional Assessment

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limited meanders. The surrounding vegetation is primarily southern Rocky Mountain mixed conifer and Douglas fir (Pseudotsuga menziesii) forests at the higher elevations, and ponderosa pine (Pinus ponderosa) at the lower elevations, although many different species of conifer occur together within the canyons themselves. Riparian species include willows, aspen, and narrow-leaf cottonwood (Populus angustifolia). There are also large aspen stands on the plateaus above the canyons in this section. 3. Mancos Valley reaches, generally between 2000-2300 meters (6500-7500 feet; Figure 2C). The Mancos Valley is relatively flat and the river in this section has a medium gradient. The flood plain is relatively wide, particularly in the middle sections of the Valley. Most of the reaches in this section have been channelized, and many have small levees along one or both banks. Remnant fluvial landforms in the flood plain indicate that there were originally numerous meanders in the past prior to channelization. The bottom of the channel is primarily medium to large cobble that has been washed down from the surrounding mountains; because the cobble “armors” the bottom there has been little downcutting of the river in this section. In a few areas, particularly in the lower parts of the valley, the channel rests on bedrock, including the Mancos Shale formation. The surrounding soils in the valley are composed primarily of gravels and alluvial deposits. The Valley is used at the present time primarily for agriculture, although the number of houses in the town of Mancos and elsewhere is rapidly increasing. The original vegetation was probably primarily piñon-juniper woodland (Pinus edulis andJuniperus spp.) with scattered native grasslands. Riparian vegetation in this section includes narrowleaf and Freemont (P. fremontii) cottonwoods. There are numerous willows and a few buffaloberry (Shepherdia argentea) in the lower parts of the valley, but most of the native riparian shrub community that would be expected to occur at this elevation is absent. 4. Lower mesa and canyon reaches, between 1615-2000 meters (5300-6500 feet; Figure 2D). This section consists of Mancos Canyon. Numerous small side canyons and ephemeral washes enter the river from the surrounding mesas. We only conducted one survey in this section, at the boundary between Mesa Verde National Park and the Ute Mountain Ute Indian Reservation. The gradient at that site was relatively low, and the channel was wide and shallow, with considerable sediment and only small cobbles on the bottom. The river banks consisted primarily of fine and medium sediments, and there was considerable meandering of the channel within the flood plain. Native riparian vegetation included willows and Fremont cottonwood, and junipers were common on the side of the canyon. The conditions at the survey site are probably typical of most of Mancos Canyon. The Park had undertaken a program to remove saltcedar from the floodplain in this area; however this exotic species appeared from the overflight to be common in the rest of the Canyon. 5. Lower elevation reaches, from 1400-1615 meters (4600-6500 feet; Figures 2E and F). This section is between the mouth of Mancos Canyon and the confluence with the San Juan River. No surveys were conducted in this section. The river has a low gradient, and the channel is wide and shallow, with a number of meanders. Most of the banks consist either of exposed bedrock or fine sediments, including wind-blown sand. The surrounding vegetation is Great Basin desert shrub. The primary riparian vegetation appears to be willows and saltcedar, although this was not confirmed with field surveys. . Major human uses and impactsNative Americans have used most of the Mancos River watershed for agriculture, hunting and fishing, and various other purposes for many thousands of years. At the present time, most human activities are focused in the upper parts of the watershed. Intensive settlement and the modification of the Mancos River and its tributaries for irrigated agriculture began around 1876, and major water delivery systems and several small water storage reservoirs had been established by the 1890s. The US Bureau of Reclamation estimated that in 1994 approximately 14,900 acres in the Valley and surrounding areas were used for agricultural production, including alfalfa, grasses and small grains. At that time, 11,700 acres were irrigated: 9900 acres by flood irrigation and 1800 acres with sprinklers (USBR 1994, cited in Yochum 2004). To deliver water to the fields, there are approximately 46 water 9