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I 3 I STUDY AREA DESCRIPTION Study Reach Description I Drainage Basin Characteristics The study reach consists of two sub-reaches: one on South St. Vrain Creek and the other on Middle St. Vrain Creek (see Index Map, Plates 2, 3 and 4). The South St. Vrain Creek reach starts from the mouth (confluence with North St. Vrain Creek) as its lower limit and extends approximately 10 miles to the confluence with Middle St. Vrain Creek as its upper limit. Throughout this study reach, it is a front range mountain stream: rather narrow in width and steep in slope (160 feet per mile), except in the downstream portion near Lyons. The Middle St. Vrain Creek reach starts from the mouth as its lower limit and extends approximately eight and one-half miles to Peaceful Valley as its upper limit. It is also a typical mountain stream, with an even steeper slope (about 240 feet per mile). I South St. Vrain Creek is located in Boulder County, Colorado. Its headquarters originate on the eastern slopes of the continental divide at elevations in excess of 13,000 feet above mean sea level. The creek flows in a generally northeastward direction and joins North St. Vrain Creek at Lyons to form St. Vrain Creek, which drains into the South Platte River near Platteville, Colorado. I I I I This study includes the entire South St. Vrain Creek basin which en- compasses a drainage area of approximately 92 square miles (see Basin Map, Plate 1). The major tributary included in the basin is Middle St. Vrain Creek which drains an area of 32.4 square miles into South St. Vrain Creek at about two miles east of Riverside. Drainage areas at different locations in the basin are given in Table 1. I Location South St. Vrain Creek at mouth South St. Vrain Creek immediately downstream of confluence with Middle St. Vrain Creek South St. Vrain Creek immediately upstream of confluence with Middle St. Vrain Creek Middle St. Vrain Creek at mouth Middle St. Vrain Creek at Peaceful Valley (State Route 72 bridge) Area (square miles) 92 66.7 The stream valleys in the study area are rather narrow in width. For this reason, early development was mainly limited to roads and bridges. More recently, however, homes have been built on the banks of Middle St. Vrain Creek at Riverside, Raymond and Peaceful Valley, some within the floodplains. With this type of floodplain development, flood hazards to the communities will continue to increase unless adequate and sound floodplain regulations are adopted. I I Table 1 Drainage Areas I I I 32.4 18.7 Obstructions to flood flows within the study reach include natural ob- structions such as trees, brush and other vegetation growing along the streambank and manmade obstructions such as buildings, bridges and cul- verts. During floods, these obstructions impede flood flows and cause backwater conditions that may increase the flood heights upstream of the obstruction and velocities downstream of the obstruction. Brush washed out during floods is carried along downstream and may collect at bridges and in culverts due to channel constrictions, thus creating a damming effect and overbank flow. As flood flows increase, masses of debris may break loose and cause a wall of water and debris to surge downstream until another obstruction is encountered. In some instances, debris may collect to the point where structural capability is exceeded I 34.3 I The mean annual precipitation over the basin is approximately 22 inches, of which 50 percent occurs during the period May through September. Mean temperature ranges from 700 F in July to 25 degrees in January, with an annual average of about 450 F. The basin topography is typical m~untain terrain covered with forest growth. I I I