During October 2003, Phase I of an instream habitat improvement project was implemented on the Dolores River below McPhee Reservoir in southwestern Colorado. The overall project site is located on mitigation lands acquired for the Dolores Project (McPhee Reservoir and facilities) and managed by the Colorado Division of Wildlife (now Colorado Parks and Wildlife) and United States Forest Service. The first phase of the Dolores River project began to address the degraded in-channel and floodplain habitats on Division-owned lands below McPhee Dam that have been affected by historic agricultural operations and flow modification from the dam project.
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History: The pre-settlement flow regime in the Dolores River through the reach below McPhee Reservoir was characterized by high spring runoff flows which tapered to little or no flow by late summer. This created a large channel with significant riparian habitats of mixed deciduous forest comprised primarily of cottonwood, willow, and box elder. In this `natural' condition, the alluvial system was allowed access to the entire valley bottom, and was characterized by a dynamic stability that allowed for modest rates of erosion and deposition that over time, maintained the river's floodplain and in-channel habitats. Because of the low late-season flows, the river probably did not support perennial occupation by native trout.
Anglo-European settlement introduced agricultural operations to the system, and with it, perturbations to the channel including stream stabilization, water diversions, and cattle grazing. Much of the floodplain was harvested for timber, then converted to pasture; these pastures were irrigated by water diversions, and cattle were introduced as the main commercial `product' of agriculture. Since the late 1800s, significant water development has occurred upstream of the reach. Combined diversions to satisfy beneficiaries of the Dolores Project (McPhee Reservoir and facilities) and senior water rights of the Montezuma Valley Irrigation Company (MVIC) average over 200,000 acre-feet per year applied to over 50,000 acres of land. All of this water is diverted out of the Dolores River at McPhee Reservoir and does not return to the Dolores River drainage.
One of the peripheral benefits of the Dolores Project was the ability to manage flows dedicated for the benefits of the downstream fishery. The benefits of perennial, cold water releases below reservoirs for fisheries have been realized in many western locations (e.g., Flaming Gorge, Navajo, Spinney Mountain reservoirs). Although there have been some coldwater fishery benefits, available cover and especially spawning habitat is extremely limited below McPhee Dam due to an over-wide and sediment limited system. Thus the downstream fishery has not benefited to the extent envisioned during planning for the Dolores Project.
Project Objectives and Design: The overall project objective is to re-establish a river morphology appropriate for the modified flow regime that will improve degraded aquatic and riparian habitats and restore natural river processes. These processes, now absent from much of the degraded reach, include pool scour, point bar formation, and floodplain rejuvenation, all vital for healthy aquatic and riparian ecosystems in this setting. Given the shallow river gradient, channel confinement (limited floodplain access), altered flow regime, reduced sediment loads and embeddedness of the cobble substrate, channel width will not decrease "naturally" through the degraded reach. Thus the restoration project aimed at reducing width, restoring channel sinuosity, and creating the types of instream habitat variability necessary for self-sustaining populations of trout (Figure 1). The following specific objectives were developed for the Dolores River habitat improvement project:
Reduce channel width (reduce width/depth ratio)
Increase over bank flooding
Enhance riparian habitat
Decrease summer water temperature
Increase pool habitat
Increase overhead structure
Phase I Project Implementation: The first 1500 feet the overall reach plan was implemented during Phase I. The primary component implemented through this reach entailed reducing channel width by importing nearly 6000 yds3 of cobble from pool excavations and from an on-site borrow area, while creating the structure necessary to maintain the narrowed bankfull channel (Figures 2). In addition, pools were excavated, and specific habitat features (such as rock clusters and `gapped' rock vanes) were built to increase cover variability and to perpetuate pool scour (Figure 3). Re-establishing floodplains within the entrenched channel will restore alluvial processes (such as point bar and floodplain deposition, and pool scour), improve vegetative recruitment, and over time, increase both channel stability and overhead structure. Phase II is planned for late summer/ early fall in 2004, and will entail more structural improvements and less placement of in-channel cobble.