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0~2204 - 17 - Sedimentation Investigations The only previous records of sediment production in the general vicinity of the watershed consist of unpublished suspended load data collected by the U. S. Corps of Engineers on the Purgatoire River at Trinidad, Colorado. Since these data relate to a much larger drainage area than that being considered in the Fishers Peak _ Carbon Arroyos Watershed, they were not used in the present study. In order to establish sedimentation rates at the proposed floodwater-retarding reservoir sites, a reconnaissance was made of the entire watershed area and the extent of the different types of sediment source areas was determined. This investigation revealed that most of the sediment produced in the watershed originates from gully and channel erosion on the high terrace and shale derived soils in the middle and lower portions of the watershed. An annual rate fOr this type of erosion was determined using 100 years as the approximate time during which recent accelerated erosion has taken place in this area. Data on past sediment and erosion damages were obtained primarily from interviews with individual landowners and officials of the City of Trinidad, Estimates of future rates of floodplain damage from erosion and sedimentation, both with and without a watershed project, were based on the size and frequency of flood flows to be expected during the life of the project. In computing future rates of sedimentation, consideration was given to the amount of reduction in sediment production that will result from proposed land treatment measures to be installed in the watershed. It is estimated that these measures will reduce the capacity required for sediment storage at the proposed reservoirs by an average of about 10 per cent. Consideration was given to the fact that some land treatment measures in this plan will become effective immediatwy after construction, while others will require several years from the start of the project to become effective. A sediment trap efficiency of 90 per cent was assumed for both floodwater- retarding structures. . Future rates of sedimentation at the proposed reservoir sites with the project installed were determined as being 0.47 acre-foot annually per square mile of drainage area at site FPC-l and 1.18 acre-feet at site FPC-2. Capacity equivalents for sediment storage in the proposed reservoirs, as finally estimated, were 0.44 and l.l inches respectively (Table 3).