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I I I I I I I I I I I I I I I I I I I measured by the USGS at St. Vrain Creek near Lyons from October 1976 to February 1981 ranged from 21 mg/l to 85 mg/l. The USGS gage station is located approxi- mately 1/2 mile upstream of the Palmerton Ditch headgate. Thus TDS levels measured in the St. Vrain Creek would appear to be very low in comparison to TDS levels measured in McCall Lake. Measured TDS levels in McCall Lake were pro- bably higher than what can normally be expected because water has not been deli- vered into or released from the lake in the past 6 years. Future diversions into and out of the lake will dilute the TDS levels making the lake water accep- table for irrigation of all crops. Present diversions from the lake would be acceptable for the majority of all crops, especially when McCall Lake water is mixed with Longmont Supply and Oligarchy Ditch water. Therefore, future water releases are anticipated to be acceptable for irrigation use. 2. Raw Water Quality, North St. Vrain Creek The water in St. Vrain Creek at the City of Longmont's North Water Treatment Plant pipeline diversion point is excellent. This is the water that the City will be exchanging for releases from McCall Lake. Average monthly values of selected water quality parameters from January 1986 through September 1987 are summarized in Table 5.9. Average values of pH, Hardness, Flouride, Manganese and Iron are well below maximum values allowed by the Environmental Protection Agency for primary and secondary drinking standards. The proposed exchange for McCall Lake water will not effect the quality of water available at the City of Longmont's North and South Water Treatment Plants. 3. McCall Lake Sedimentation An estimation of the average annual sediment inflow to McCall Lake was made by determining the difference in storage capacity from the original 1909 topographic map and the December 1986 topographic map. The storage capacity at -21-