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The ladder alignment is also shown 20 feet east of the existing dam with the intent of providing sufficient clearance between the fish ladder and the existing dam to avoid disturbance to the existing dam structure and foundation during construction of the fish ladder. 6.1.3 Elevations The upstream ladder invert elevations, set at 4950, take advantage of a low spot located on the east (left) bank of the Gunnison River. This low spot is approximately 3 feet lower than the top of the dam. Although the dam has been in place for over 100 years, it appears that sediment accumulation is not occurring on the upstream side of the dam. This may be due to the high permeability of the dam material (rock cribbing), which allows for the passage of fine materials. This portion of the Gunnison River is also below several dams, which may result in flows with little sediment. The invert of the downstream ladder inlet (fish entrance) is set at elevation 4948 which, based on the October 1999 survey data, appears to be the dominant channel elevation at the toe of the dam. The entrance is located near a small, 2 feet deep scour hole. Because the scour hole could create unusual localized flow patterns, this alternative includes filling the scour with riprap to the elevation of the surrounding river bed. The placement of the riprap also elevates the hole to match the elevation of the ladder entrance, which helps the target species locate the fish ladder if the fish are swimming along the channel bottom. The floor elevations of the ladder also maintain 2 feet minimum depth of water in the fish ladder during the time period when it is anticipated that the passageway is in use, in particular late April through August, which is represented by flows ranging from 3000 cfs to 350 cfs, respectively. Based on the channel surveys in October of 1999 and the hydraulic calculations performed for this analysis, the elevation difference between the channel bed upstream of the dam, where the fish ladder inlet is proposed, and the calculated water surface elevation for 350 cfs is approximately 3.4 feet, while downstream the water depth in the ladder 25 I I I I I I I I I I I I I I I I I I I