2020-06-04_PERMIT FILE - C1981008A (2)

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irrigation tail water provided a dilution effect. Little variability between the seasons for pH was detected at SW -N103. During irrigation, pH averages 7.84 and ranges from 6.84 up to 8.10 units. After the irrigation season, pH averages 7.88 and ranges between 7.50 and 8.06 units. Similar to previously discussed surface water monitoring sites, the mean of TDS during the non -irrigation season was more than double (3,135 mg/1) the mean for this constituent during the irrigation period (1,371 mg/1). Again, the higher levels of the major ions in Calamity Draw at this site after irrigation stops explains this trend. At Site SW -N103, both TSS and FeT averaged higher during the irrigation period (57.0 mg/1 and 1.46 mg/1, respectively) than after irrigation has ceased (39.4 mg/l, TSS and 1.05 mg/1 FeT). The mean value of FeD was slightly higher (.04 mg/1) during the non -irrigation period compared to the average FeD (.02 mg/1) determined for the irrigation season. As seen for all previously discussed sites, MnT averaged consistently higher (1.24 mg/1) after the irrigation season ended than during the irrigation period (.33 mg/1). West Lateral Irrigation Ditch - Flow and Water Ouality The West Lateral irrigation ditch is part of the Colorado Co-operative Company's main ditch system that originates on the San Miguel River just upstream of Cottonwood Canyon (about 12 miles upstream of Naturita). Construction of the main ditch commenced in 1896, and water began flowing to the First Park area (see Map 2.04.7-1A) via the ditch in the spring of 1904 (Mercer, 1967). The main ditch, as well as the main laterals, is basically an unlined open -channel excavated in shallow soils and sandstone. Where the ditch crosses drainages, pipes and aquaducts have been constructed to carry the flow. The main ditch has an average gravity run of four feet per mile from the San Miguel River to the First Park area (Mercer, 1967). For a more historical discussion on the Colorado Co-operative Company's development of the regional ditch system, the reader should refer to Section 2.04.3, Site Description and Land Use Information. The West Lateral splits off the main ditch near the northern boundary of the Nucla Townsite. The West Lateral follows a ridge -top course westward from Nucla to the main north -south county road, then winds south and southwest until the conveyed irrigation water is used by the various shareholders located along the ditch's course. A total of 18 diversions have been identified on this lateral; many consist of wooden "splitte boxes" that divert a consistent proportion of the irrigation water from the lateral to fields via small ditches and channels. Some diversions simply consist of pipes and hoses that also route irrigation water to slightly lower elevation fields. The West Lateral irrigation ditch is an unlined open channel. Where the ditch courses on the ridge -top separating Tuttle Draw to the north and the Calamity Draw Valley to the south, the ditch has been excavated through shallow soils into the highly fractured and weathered sandstone. Over time, the ditch channel -bottom has been subsequently scoured and eroded, resulting in significant reaches of exposed and fractured sandstone. Further down the ditch course, the ditch channel is constructed through deeper soils, and features a sandy channel bottom. In most reaches, vegetation grows so heavily on the banksides that the Colorado Cooperative Company performs routine cutting and cleaning to ensure the integrity of the ditch channel. Occasionally, where the ditch courses through the deeper soils, the ditch bottom is periodically cleaned out (dredged). The overall gradient of the West Lateral irrigation ditch is approximately .0 14 ft/ft. Flow in the West Lateral irrigation ditch is exclusively controlled by the Colorado Cooperative Company. Page 2.04.7-22 March 2017 (TR -77)