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I. I II I. ! I I I I ,1. ij I \1 I , ;. I i I 11 ~ , ! \1 ! I I I . l The average annual flow in the Gunnison River is about 1,360 cfs at the Gunnison Tunnel, which is over five times greater than the average annual flow of the Uncompahgre River near Colona, just upstream of the project boundaries. Historically, both rivers carry high natural flows during late spring and early summer resulting from snow melt. Natural flows on both rivers are now regulated by dams operated by the Reclamation which are upstream from project lands. Both rivers would play an important role in providing water supplies for the hydropower project. The two rivers have been linked by a network of canals and laterals since the early 1900's. The major features of this system (see Figure 2.1) include the Gunnison Tunnel, throu9h which water is diverted from the Gunnison River into UVRP lands, and the South Canal, which conveys diverted flows from the Gunnison Tunnel into the Uncompahgre River. A small amount of flow is diverted from the South Canal to meet demands along that system, including those of the AB Lateral and the West Canal, but the majority of the over 336,000 acre-feet which are annually diverted into the canal are combined with Uncompahgre River water to irri9ate UVRP lands. Water is diverted from the Uncompahgre at six diversion dams and passed through several hundred miles of canals, laterals, and drains to meet irrigation needs. About half of this flow is used consumptively (mainly for irrigation), and the remainder flows northward towards Delta, where the Uncompahgre jOins the Gunnison River. Table 2.1 presents average annual flows (based upon the study period of 32 years) which are diverted for irrigation on UVRP lands. Mean monthly flows for each of the canals are shown in Appendix A of this report. Over the 32-year study period, available irrigation water supplies from the Gunnison and Uncompahgre Rivers have averaged slightly more than 505,000 acre-feet per year. However, the annual UVRP-related irrigation and winter stock water demands have averaged about 536,635 acre-feet per year. Given that the annual flow in the Uncompahgre River has averaged only about 168,000 acre-feet, the importance of the Gunnison River in meeting demands can be readily seen. Further, because the average demands have exceeded the average supplies, the importance of inflows from small tributaries of the Uncompahgre River and reuse of irrigation return flows can also be seen. 2.1.1 South Canal The South Canal is the principal conveyance structure for transporting flows from the Gunnison River into the Uncompahgre River. The canal begins at the West Portal of the Tunnel and runs southwesterly about 11.4 miles to the Uncompahgre River. The Canal has a maximum carrying capacity of about 1,010 cubic feet per second (cfs) at the upper end; however, the capacity is reduced to sli9htly more than 750 cfs near the river. About half of the Canal's length is lined with concrete in either rectangular or trapezoidally-shaped channels. Flows in the canal are monitored by the UVWUA at a gaugin9 station located about one mile downstream of the West Portal. Records for this gauge and other key points in the UVRP system are presented in Appendix A of this report. - 2.4 - f' ~ . . .