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Geomorphology <br />Dolores River Dialogue Geomorphology Analysis <br />Gigi Richard and Andrew Wilcox <br />Introduction <br />Physical processes in rivers reflect a set of interactions between flow, sediment transport <br />and chaiuiel morphology, and these in turn combine to form the physical template of aquatic <br />habitat. River channel morphology reflects along-term balance between a river's driving forces, <br />including water discharge and channel gradient, and its resisting forces, including the size of <br />sediment in the bed and banks, the frequency of bedrock exposures, bedform characteristics, <br />riparian vegetation, channel curvature or sinuosity, and floodplain characteristics. Changes to a <br />river's flow and/or sediment regimes, as may occur due to human uses of river systems, can alter <br />these driving or resisting forces and produce adjustments in channel form and associated aquatic <br />habitat. In particular, changes in river flows, sediment supply, or other controlling variables can <br />alter channel dimensions, flow velocities, bed gradients, bed and bank sediment sizes and <br />mobility, channel pattern (e.g., meander characteristics), and bedforms (e.g., distribution of pools <br />and riffles). Because of the fundamental linkages between river morphology, aquatic habitat, and <br />anthropogenic modification of river flow and sediment regimes, geomorphology, the study of <br />landforms and the processes that shaped them, provides a valuable avenue for investigating the <br />Dolores River downstream of McPhee Dam. <br />A preliminary assessment of geomorphology issues in the Dolores River downstream of <br />McPhee Dam is presented below. Tlus includes analysis of the following: (1) current and <br />historical geomorphic conditions, (2) effects of flow modification on geomorphic processes, (3) <br />sediment dynanucs in the Dolores River, including both dam-related and non-dam related human <br />effects; and (4) data gaps and future directions. This assessment is based primarily on previous <br />reports, supplemented by limited analysis of aerial photographs and field observations, and is <br />designed to provide information relevant to future management of the Dolores River. Further <br />field investigations and historical analysis would be needed in order to develop a complete <br />geomorphic assessment of chamlel conditions and changes compared to both pre-dam and <br />presettlement <br />conditions. <br />Geomorphic conditions in the Dolores River downstream of McPhee Dam <br />The defining characteristic of the Dolores River downstream of McPhee Dam is the series of <br />steep-walled canyons, separated briefly by broad structural valleys, through which the river <br />flows. Within these canyons, the river is flanked by steep sandstone cliffs and slopes with very <br />shallow soils and narrow valleys and floodplains (CO DNR et al., 1976; USDA, 1972). Many <br />segments of the canyon reaches of the Dolores River are likely bedrock-controlled, meaning that <br />bedrock exposures in the channel bed and banks are common and that only relatively thin <br />veneers of sediment would be present over the underlying bedrock over a longterm time scale. <br />Another notable characteristic of the Dolores River is that because of the large elevation and <br />19 <br />