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At Site SW-N103, both TSS and Fer averaged higher during the irrigation period (57.0 mg/1 and <br />1.46 mg/I, respectively) than after irrigation has ceased (39.4 mg/I, TSS and 1.05 mg/I FeT). The <br />mean value of Fep was slightly higher (.04 mg/1) during the non-irrigation period compared to the <br />average Fep (.02 mg/1) determined for the irrigation season. As seen for all previously discussed <br />sites, MnT averaged consistently higher (1.24 mg/1) after the irrigation season ended than during <br />the irrigation period (.33 mg/1). <br />West Lateral Irrigation Ditch - Flow and Water Quality. The West Lateral irrigation ditch is part <br />of the Colorado Co-operative Company's main ditch system that originates on the San Miguel River <br />just upstream of Cottonwood Canyon (about 12 miles upstream of Naturita). Construction of the <br />main ditch commenced in 1896, and water began flowing to the First Park area (see Map <br />2.04.7-1A) via the ditch in the spring of 1904 (Mercer, 1967). The main ditch, as well as the main <br />laterals, is basically an unlined open-channel excavated in shallow soils and sandstone. Where <br />the ditch crosses drainages, pipes and aquaducts have been constructed to carry the flow. The <br />main ditch has an average gravity run of four feet per mile from the San Miguel River to the First <br />Park area (Mercer, 1967). For a more historical discussion on the Colorado Co-operative <br />Company's development ofthe regional ditch system, the reader should referto Section 2.04.3, Site <br />Description and Land Use Information. <br />The West Lateral splits off the main ditch near the northern boundary of the Nucla Townsite. The <br />West Lateral follows a ridge-top course westward from Nucla to the main north-south county road, <br />then winds south and southwest until the conveyed irrigation water is used by the various <br />shareholders located along the ditch's course. A total of 18 diversions have been identified on this <br />lateral; many consist of wooden "splitte boxes" that divert a consistent proportion of the irrigation <br />water from the lateral to fields via small ditches and channels. Some diversions simply consist of <br />pipes and hoses that also route irrigation water to slightly lower elevation fields. <br />The West Lateral irrigation ditch is an unlined open channel. Where the ditch courses on the <br />ridge-top separating Tuttle Draw to the north and the Calamity Draw Valley to the south, the ditch <br />has been excavated through shallow soils into the highly fractured and weathered sandstone. Over <br />time, the ditch channel-bottom has been subsequently scoured and eroded, resulting in significant <br />reaches of exposed and fractured sandstone. Further down the ditch course, the ditch channel is <br />constructed through deeper soils, and features a sandy channel bottom. In most reaches, <br />2.04.7-39