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.- . ST. VRAIN CREEK BASI~I The primary drainage basin in Boulder County is that of St. Vraln Creek. The incorporated towns along its banks include Lyons and Longmont. Many historic townsites and new subdivisions lie within this basin. Flood dates for the areas in the St. Vrain Basin vary for upstream to downstream locations. The flood of record for Lyons is 1941; that of Longmont is 1921. This difference is caused by two factors: localized storms and the additional stream water discharge of Lefthand Creek at Longmont. Lyons Located at the confluence of North and South St. Vr;]in Creeks, Lyons was settled in the early 1860's, became an estilblished townsite in 1882, and was incorporated in 1891.137 A stream gauge, located about one half nile downstream from the confluence of North and South St. Vrain Creeks, operated from 1887 to 1891 and from 1895 to the present.138 The Longr.1ont and Buttonrock reservoirs lie upstream from the community on the North St. Vrain. They, like Barker Reservoir in the Boulder Creek Basin or Gross Reservoir in the South Boulder Creek Basin, do not have storm water detention capacities. That is, they are not designed for flood control.139 . Major floods in Lyons identified by previously published research include:140 1864 - June 1946 - July 18 1876 - May 1947 - June 17 1894 - May 31-June 2 1949 - June 4 1919 - July 30 1951 - August 3 1921 - June 7 1957 - May 9 1924 - June 14 1961 - June 3 1930 - August 10 1967 - August 30 1935 - r.,lay 27 1969 - May 7 19~1 - June 22 Of these events, the 1894, 1919 and 1941 floods have been considered the most severe occurrences.141 The discharge measured or calibrated at Lyons for those floods was 9,800,9,400 and 10,500 cubic feet per second [cfs), respectively.142 The 1941 flood is often regarded as the largest of the three. However. the estimated difference between the 1894 and 1941 floods was only 700 cfs. . As noted, the Lyons stream gauge was not in operation in 189~, when the estimated peak discharge was 9,800 cfs. In addition, the comparison of .slope area measurements from one event to another is not exact due to the changing character of the stream and the different methods of calibration used. The gauge near the confluence of north and South St. Vrain Creeks was in operation from 1395 until the