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C;imsent Agenda Item 2 ~eptember 2003 Board Meeting Page 2 of 4 tpe donated right to TCF. Staff recommends that the Board authorize the CWCB Director to sign the . ~ttached agreement l{iscussion rhe water right donated to CWCB consists of 16.38 cfs out of a total 20 cfs of the Snowmass Divide 1;>itch Enlargement water right (donated right). The Snowmass Divide Ditch diverts water from ~nowmass Creek, and irrigates lands in both the Snowmass Creek drainage and the Roaring Fork drainage on the east side of the Watson Divide. The donated right was used on 20 acres of land in the ~nowmass Creek drainage, and an additional 220 acres in the Roaring Fork basin. Spronk performed a ~tream depletion analysis for the donated water right, based on a historical study period of 1950 through 1996. The net average annual stream depletion for the donated right was estimated to be 28 acre-feet. This number is relatively low as compared to average annual diversions of 1160 acre-feet, because the ~enior 6.2 cfs Snowmass Divide Ditch water right, which'remains on the land, is generally sufficient to Ip.eet the consumptive crop water demands on the ranch. 1;>ue to the geography and layout of the irrigation system, the historical exercise of the donated water right resulted in a large stream depletion to Snowmass Creek, because a majority of the diversions were (lxported across Watson Divide to the adjacent Roaring Fork Basin. The stream depletion to Snowmass ~reek averaged 985 acre-feet per year, which occurred, during the diversion season, generally May through September. Return flows accrued to Snowmass Creek from Ditch seepage and irrigation, which qontributed to flows in October through December. The importation of water to over the divide to the Roaring Fork Basin created a significant accretion to the Roaring Fork in the form of irrigation return . f,lows. This accretion averaged 958 acre-feet per year, and occurred year round. At the confluence of ~nowmass Creek and the Roaring Fork River, the average historical depletion to Snowmass Creek of 985 ~cre-feet is mostly cancelled out by the average historical accretion to the Roaring Fork of 958 acre-feet, Ipaving a net depletion of 28 acre-feet. Ip. summary, the exercise of the donated right depleted Snowmass Creek streamflows during the irrigation season but provided some benefit to those flows ,in the fall and early winter months from return f1ows. The exercise of the donated right benefited the Roaring Fork River flows downstream to the Qonfluence of Snowmass Creek on a year round basis, due to delayed irrigation return flows. the CWCB holds instream water rights on Snowmass Creek and the Roaring Fork. Roaring Fork River ipstream flow rights in the affected reach are 55 cfs from April through September, and 30 cfs from q>ctober through March. Snowmass Creek instream flows, total 15 cfs from April 1 through October 15, ~d 12 cfs for the remainder of the year for the affected reilch downstream to the confluence with Capitol ~reek. Instream flow rights below Capitol Creek downsj:ream to the Roaring Fork River are 22.5 cfs ftom April 1 through October 15 and 11 cfs for the remainder of the year. ~pronk performed an analysis of historical streamflows in the Roaring Fork River and Snowmass Creek tp assess the frequency of times when the CWCB's instream flow rights are likely to not be satisfied. ~oaring Fork flows were evaluated based on gage flows at the USGS gage designated as "below Maroon q;reek". The period ofrecord for this gage was 1914 through 1918 and 1991 through 1999. Over this period there were no shortages to the instream flow right. ' I!'or Snowmass Creek, due to the absence of historical gaged flow records, it was necessary to estimate ~ows based on hydrological modeling techniques. Spronk used gaged daily flows from Maroon Creek as Flood Protection. Water Project Planning and Financing. Stream and Lake Protection Water .Supply Protection. Conservation Planning .