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Terrace Reservoir has created new geomorphic conditions upstream and downstream of the reservoir. <br />The decreased sediment load downstream of Terrace Reservoir has slowed the natural evolutionary <br />process in which the channel in the alluvial fan becomes choked with sediment, and it avulses across the <br />fan. Under the altered conditions, channel aggradation upstream of the reservoir is caused by the <br />deposition of sediment as the river enters the reservoir. Degradation may occur downstream of Terrace <br />Reservoir as flows exit the reservoir, due to the "hungry water" effect. "Hungry water" occurs because <br />the river has the capacity to carry a given sediment load but is not carrying this load because the <br />sediment has dropped out in the reservoir. However, it was very difficult to verify this degradation <br />below the reservoir during the site visit. <br />Available rating curves for gages on the Alamosa River were obtained to determine if the channel has <br />aggraded or degraded over the period of record. There are two long-term gages on the Alamosa River <br />which were used to determine historical changes. Long-term rating tables dating back to 1934 were <br />obtained from the USGS and Colorado Division of Mater Resources (CD~XIR) for the stream gages <br />upstream and downstream of Terrace Reservoir. The rating curves upstream of the reservoir show the <br />channel being relatively stable between 1934 and 1954 with slight degradation between 1954 and 2002 <br />(Figure 2-11). This slight degradation indicates the gage may be far enough upstream (7,000 feet) to <br />avoid being affected by the reservoir backwater conditions. Usually reservoir backwater conditions <br />would cause aggradation due to settling of suspended sediment. The rating table downstream of the <br />reservoir does not show the degradation expected below a reservoir (Figure 2-12). Usually the clear <br />water from a reservoir outlet causes the stream to pick up sediment causing degradation, unless the <br />channel is made of rock. The downstream rating table indicates channel aggradation between 1934 and <br />1956 and relatively stable conditions from 1956 to 1988. The channel bed aggradation suggests the river <br />has picked up its sediment load by the time it reaches the gaging station which is located approximately <br />2,500 feet downstream of Terrace Reservoir. <br />Figure 2-11. Rating Curve Above Terrace Reservoir <br />Rating Curves Above Terrace Reservoir <br />3000 <br />2500 <br />y 2000 1934 <br />~ 1500 ~ 1954 <br />3 X1967 <br />_o <br />~ 1000 - 2002 <br />500 <br />0 <br />1.5 2 2.5 3 3.5 4 4.5 <br />Gage Height (ft) <br />Alamosa River Watershed Restoration Master Plan and Environmental Assessment Page 2-15 <br />