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Figure 2-12. Rating Curve Below Terrace Reservoir <br />Rating Curves Below Terrace Reservoir <br />1800 <br />1600 <br />1400 <br />N 1200 1934 <br />v 1000 X1956 <br />3 800 ~ 1966 <br />0 <br />~ 600 1988 <br />400 <br />200 <br />0 <br />1.5 2.5 3.5 4.5 5.5 <br />Gage Height (ft) <br />2.2.6 Terrace Reservoir to Terrace Main Canal (Reach 4) <br />This reach of the Alamosa River is confined by steep valley walls. The floodplain through this reach is <br />150 feet wide. The channel meanders across the entire valley. The channel sinuosity of this reach is 1.3, <br />and there is an average channel slope of 0.8%. This reach marks the beginning of irrigation diversions. <br />There are two ditches within this reach, which have an appropriation to divert a total of 147.02 cfs, or <br />19 percent of the two-year event, which is 761 cfs. <br />Irrigation diversions have a significant impact on the amount of water in the Alamosa River channel, <br />which in turn impacts river channel characteristics and the ability of the channel to convey its sediment <br />load. As discussed in Section 2.3, 36 ditches divert water from the Alamosa River. During the irrigation <br />season, there is typically not enough water to fill all irrigation water rights (Vandiver, 2003). <br />As flows are reduced downstream of each irrigation headgate, sediment typically drops out, resulting in <br />bed aggradation. Less flow translates to reduced stream power and decreased ability to convey sediment. <br />This aggradation creates an unstable channel reach with widening and meandering expected for some <br />distance until the channel adjusts to the new flow. If the diversion includes a dam, such as the Terrace <br />Main Canal, deposition would also be expected upstream of the diversion due to the flattened slope and <br />artificial grade control. <br />Analysis of the photo logs between Terrace Reservoir and Highway 285 (Agro Engineering, 2003, Black <br />Creek Hydrology, 2002, and Rosgen, 1999) shows the channel changes at many of the diversion <br />structures. 1Vlany photos, such as the following two (Figure 2-13 and Figure 2-14), show unstable <br />channels with eroded banks, and bed aggradation at the diversion structures. Adjustment in the Alamosa <br />River channel geometry becomes more pronounced as more water is diverted from the river in the <br />downstream reaches. <br />Alamosa River Watershed Restoration Master Plan and Environmental Assessment Page 2-16 <br />