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2.2 Channel of Alamosa River and Major Tributaries In developing a solution for a particular river problem, it is important to evaluate the entire river system and its environment. The river, its floodplain, and the entire watershed are interrelated. Changes to one aspect affect all others. The stream's slope, width/depth ratio, channel sinuosity, entrenchment ratio, sediment types, sediment loading, hydrology, and channel hydraulics are all related and have an impact on how the stream functions. The preceding list of attributes can be divided into three major categories: sediment loading, slope, and discharge. All of these factors are influenced by the surrounding environment, and changes to that environment affect the river and its state of equilibrium. Natural conditions and human induced disturbances play important roles in the stream's function. Often the characterization of the existing system lends itself to an understanding of the nature of the system's instability. In the same manner, numerous researchers (Leopold, et al., 1964; Schumm, et al., 1987; Rosgen, 1996) have identified a series of strong relationships between stream and channel variables. For example, as stream discharge decreases (i.e., below an irrigation diversion), the ability of the channel to transport its sediment load also decreases, which in turn affects the channel slope. Maintaining the balance between sediment loading, slope, and discharge in the development of stable channel geometry is an important consideration. The Alamosa River headwaters lie in rugged mountainous terrain where tributaries transporting high sediment loads flow through steep canyons. The slope flattens considerably at the confluence with French Creek, about 4 miles upstream of the Terrace Reservoir inflow. Downstream through the reservoir, past the Terrace Main Canal ant to County Road 10, the slope is similar. ~1ith flatter slopes, the river is unable to convey the same sediment load that it could in the steep canyons. Due to the decreased stream power, the sediment load drops out, creating a large alluvial fan (Figure 2-5). Coarse sediments such as boulders and cobbles drop out first. As the slope flattens, progressively finer grained sediments are deposited. Under natural conditions, the alluvial fan segment of the Alamosa River would continually aggrade, become choked with sediment, and shift across the fan area. The natural "pre- disturbed" condition of the Alamosa River was a highly avulsive environment. Agro Engineering, Inc. conducted a bankline analysis of Reach 2 from County Road 10 to Gunbarrel Road in 2003 (Agro Engineering, 2003). Historical aerial photos were used by Agro Engineering, Inc. to show the channel plan changes between the years 1941 and 1998. The report identifies areas of channel straightening and bank erosion in this reach. Many of the conclusions on the Alamosa River channel changes and reactions were based on the bankline analysis of this section of river. Unfortunately, little other historical channel geometry information is available for the river above and below Terrace Reservoir to quantify channel changes over time. During a site visit in July 2004, Lidstone & Associates measured cross sections, slopes, and bed material to characterize the river channel in each reach from ~Iightman Fork to Highway 285. The information collected during the site visit was used to classify the Alamosa River upstream of Terrace Reservoir, where relatively little data exists, and compare to previous channel classifications between Terrace Reservoir and Highway 285. Table 2-4 summarizes the channel geometry information gathered in the site visit. Sediment information is discussed in Section 2.7. Alamosa River Watershed Restoration Master Plan and Environmental Assessment Page 2-8