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I I I I I I 'I I I I I I I I I I I I I SECTION 2 - STUDY AREA DESCRIPTION 2.2 Climate 2.1 Drainage Basin Characteristics Precipitation and temperature records are available for Longmont and Allenspark. Colorado. These locations appear to adequately describe the range of climate variation within the study area. St. Vrain Creek, a left bank tributary of the South Platte River, originates on the eastern slopes of the Rocky Mountains near the Continental Divide. North St. Vrain and South St. vrain Creeks, which drain approximately 211 square miles of mountain area. join in the foothills at Lyons, Colorado to form the main stream of the St. Vrain. From Lyons. the St. Vrain flows in a southeasterly direction through Longmont, Colorado. Lefthand Creek a right bank tributary of the St. Vrain, joins St. vrain Creek south of Longmont. Boulder Creek, a major right bank tributary, joins the St. Vrain approximately 4.5 miles downstream of Longmont. From its confluence with Boulder Creek, the St. Vrain flows 15 miles in a northeasterly direction to its confluence with the South Platte River near Milliken and Platteville, Colorado. Elevations within the basin range from approximately 4,760 feet at the mouth to over 14,000 feet on Longs Peak. The St. Vrain basin above Lyons, Colorado is primarily forested, mountainous terrain, while the St. Vrain basin below Lyons is primarily pasture and crop lands. The average annual precipitation at Longmont is approximately 13 inches. Longmont receives approximately 9.4 inches of precipitation during the months of April to September. The maximum rainfall depth accumulated over a 24-hour period was 4.04 inches, which occurred on May 9, 1957. Mean monthly high temperatures range from 89 degrees F in July to 41 degrees F in January. Mean monthly low temperatures range from 58 degrees F in July to 12 degrees F in January. The average annual precipitation at Allenspark is approximately 20.6 inches. Allenspark receives approximately 17.5 inches of precipitation during the months of April to September. The maximum 24-hour rainfall depth recorded at Allenspark is 4.3 inches, which occurred on April 2, 1957. Mean monthly high temperatures range from 76 degrees in July to 35 degrees in January. Mean monthly low temperatures range from 45 degrees in July to 14 degrees in January. This study includes the St. Vrain Creek from Airport Road (85th Street) in Longmont to the confluence of the North and South St. Vrain at Lyons, and the North St. Vrain Creek from Lyons to a point approximately 7 river miles upstream of the confluence of North and South St. Vrain Creeks. Drainage basin areas for St. Vrain Creek and its tributaries at a number of locations are listed in Table 2. Section 3. 2.3 Study Reach Description The study reach consists of two sub-reaches: one on the main stream of the St. Vrain and the other on the North St. Vrain Creek. The sub-reach on the main stream of the St. Vrain starts at 85th Street at the western edge of Longmont and extends approximately 7.5 river miles upstream to Lyons to the confluence of the North and South St. Vrain Creeks. Throughout this study reach, the banks of the St. Vrain are bordered by pasture and crop lands, with the banks themselves lined with brush and cottonwoods. The channel is irregular in cross section through the main stream study reach, consisting of high banks approximately 150 to 300 feet apart with the low flow channel meandering between these limits. The channel bed consists of shifting sand and gravel bars strewn with cobbles and small boulders. The average bed slope for the main stream study reach is approximately 40 feet of drop per mile eO.76%).. The North St. Vrain sub-reach begins at the confluence of the North and South St. Vrain Creeks and extends approximately 7 river miles upstream. The North St. Vrain is a typical mountain stream: A number of small lakes and reservoirs exist in the upper portions of the basin including Buttonrock Reservoir. Longmont's major raw water storage reservoir. Completed in 1969, Buttonrock lies on North St. Vrain approximately six miles west of Lyons. Buttonrock was constructed for municipal water supply storage and was not intended to serve as a flood control structure. However, Buttonrock does provide some residual flood control effects by attenuating peak flood flows as they are routed through the reservoir. Numerous smaller offstream reservoirs, constructed primarily for agricultural purposes are located throughout the lower St. vrain Creek basin, and provide some residual flood control benefits, but to a lesser degree than that provided by Buttonrock Reservoir. -3-