Mancos Watershed Functional Assessment

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FUNCTIONAL ASSESSMENT OF THE MANCOS RIVER WATERSHED: MANCOS VALLEY AND ADJACENT AREAS INTRODUCTION Description of the Mancos River Watershed The Mancos River originates in the western flanks of the La Plata Mountains, a western subrange of the San Juan Mountains in southwestern Colorado. It then flows southwest through the Mancos Valley and Mancos Canyon, until it joins the San Juan River in northwestern New Mexico (Figure 1). The river is 185 km or 116 miles in length, including the major upstream tributaries, and drains an area of 22 approximately 2075 km (800 miles ). The Mancos River watershed is part of the Colorado Plateau 2 geologic region, and is often divided into two main parts: an upper watershed of approximately 527 km 2 (203 miles) that includes Mancos Valley and the surrounding mountains, and a lower area that begins in Mancos Canyon at the confluence Weber Creek, and drains the mesa and desert lowland country of Mesa Verde National Park, the Ute Mountain Ute Indian Reservation, and the surrounding regions. Four main tributaries begin among the ridges and peaks of the upper watershed (the highest are Hesperus Mountain and Lavender Peak, at 4033 meters (13,232 feet) and 4036 meters (13,240 feet) respectively). They are the East, Middle and West Mancos Rivers, and Chicken Creek. A fifth major tributary, Mud Creek, drains the lower elevation regions in the northwestern part of the upper watershed. In the lower watershed, numerous small side canyons and ephemeral washes enter the river as it moves through Mancos Canyon. The river then flows through relatively flat desert county until it enters the San Juan River. Navajo Wash forms the only major drainage system that joins the river in the lower watershed, and it includes the area north of the Mancos River and between the mesa country to the east and Sleeping Ute Mountain to the west (Figure 1). Mean annual precipitation in the overall watershed ranges from over 100 centimeters (40 inches) at the highest elevations in the mountains to less than 20 centimeters (8 inches) where the Mancos River enters the San Juan River. The Mancos River travels through almost all of the major vegetation life zones found in the Colorado Plateau region, including Alpine Tundra, Sub-alpine Coniferous Forests, Spruce-Fir Forests, Mixed- Conifer Forests, Ponderosa Pine Forests, Piñon-Juniper Forests, Plains-Mesa Grasslands and Savannah (now primarily converted to agriculture), and Desert Scrublands. As a result, the biological diversity of both the aquatic and terrestrial riparian communities across the entire watershed is high. Hydrogeomorphic sections. For the purposes of conducting the Rapid Stream-Riparian Assessment (RSRA) surveys, we used an initial overflight of the entire watershed and subsequent field work to identify five major sections of the river that vary significantly in terms of their hydrology, geomorphology and ecological communities. Examples of these sections are given in Figure 2. Because the stream-riparian ecosystem would be expected to exhibit different characteristics in each of these sections, the functional condition of an individual reach in each section was scaled during the RSRA 6