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CHAPTER 2: HISTORICAL HYDROLOGY AND BIOLOGICAL COMPONENTS OF THE STUDY AREA Historic Hydrology The Yampa River Basin drains approximately 7,660 square miles. Most runoff is produced by melting snowpack in higher elevations at the eastern and southern edge of the basin (Hydrosphere 1993). The Yampa River is 180 miles long and varies in elevation from over 10,000 feet above sea level (fast) at its headwaters to 5,100 fasl at its confluence with the Green River. Natural flows in the Yampa River are seasonally variable and typically reach a peak in late spring and decline by late summer to minimal levels in late fall. Peak to baseflow ratios for annual hydrographs on the Yampa River can exceed 100 to 1. Nearly two thirds of the annual discharge in the Yampa River near Maybell occurs in May and June. Runoff typically occurs between April and July with peak flows usually occurring in mid May to early June. Baseflows typically occur in late August and early September (Figure 2.1). The hydrographs in Figure 2.1 represent the 90%, 80%, 50%, 20% and 10% exceedance flows for 82 years of flow records for the Maybell gage and are an indication of the magnitude. and frequency of historical flows. From 1916 to 1997, mean total annual runoff for the Maybell gage is 1.13 million-acre feet (MAF). The minimum flow for the 82 years of record at the Maybell gage was 3 cfs in 1934, foHowed by 7.9 cfs in 1994. Half the years had a minimum flow exceeding 128 cfs, and 20% had minimum flows exceeding 235 cfs (Figure 2.2). The year with lowest summer flows was 1934 and the wettest year was 1997 (Figure 2.3). The median flow year, based on total water volume between August 1 and October 30, was 1967. The minimum flow in 1967 was 118 cfs and the number of days flow was below 128, 150 and 200 cfs were 1, 7 and 18 respectively (Figure 2.3). Exceedance flows of 90%, 80%, 70%, 60% and 50% for the baseflow period are shown in Figure 2.4. The annual runoff for 1996 and 1997 was 1.57 MAF and 1.88 MAF, respectively (USGS records CO-96-2 page 358, CO-97-2 page 396). Bankfull flow, defined as the 1.5 year recurrence channel forming flow (Gordon et aI1992), at the Maybell gage is 8,463 cfs. The median, 50% exceedance, or 2-year peak flow recorded on the Maybell gage is 10,000 cfs and the 10-year flood is 14,634 cfs. In both 1996 and 1997 annual runoff volumes were above average with peak flows of 14,700 cfs and 16,400 cfs, respectively. According to the Colorado River Decision Support System (CRDSS) (information provided by the Colorado Water Conservation Board), during the 17 year period between 1975 and 1991, about 57% of 20