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I I I I . , I I I I I I I I I . I . I The resulting study area then included the following major sub-basins: Grange Hall main stem, Tributary S, Tributary SE, Tributary N, and Tributary NE. I. Grange Hall Creek The largest sub-basin in the study area (see Figure 11-2) is the Grange Hall Creek main stem area extending from the South Platte River to Federal Blvd., which contains 3,184 acres (4.97 square miles). The major portion of the upper reaches of the basin are in Northglenn, while the lower reaches come under the jurisdiction of Thornton and Adams County (see Table II-I). The major features of the basin are the Niver Canal, the Croke Reservoir, the Northglenn Shopping Mall, and 1-25 for the upper reaches within Northglenn. The detention reservoir at Washington Street, and the inadvertent detention obtained at the railroad and spur embankment, Colorado Blvd., and Riverdale Road are the major features in the lower reaches of the basin. The water course in the upper reaches of the basin consists of storm sewer and street flow. Open channel flow begins at 1-25 and continues to Riverdale Road in a well- defined channel. From Riverdale Road to the confluence with the South Platte River the channel has been altered by the farming in the area and is essentially a small irrigation or drainage ditch with a large overbank flow area. 2. Tributary S The second largest sub-basin in the study area is Tributary S, which is a right bank tributary to Grange Hall Creek. The sub-basin contains I, 154 acres (1.80 square miles) and extends from the confluence just downstream of 104th Avenue to approximately Zuni Street, west of Pecos Street. The area west of 1-25 and the area downstream of 104th Avenue lie within Northglenn. Between 1-25 and 104th Avenue the sub-basin lies within Thornton. The major features of the sub-basin are Huron Street, the concrete-lined Tuck Lateral, 1-25, Washington Street, four small ponds, and 104th Avenue. The water course above Huron Street consists of overland, street, and storm sewer flow. From Huron Street to the Tuck Lateral the water course consists of a reasonably well-defined open channel. Through the vicinity of the Tuck Lateral part of the storm flows are conveyed in a 60" concrete pipe and a 3Y2' to 8' concrete box culvert, which passes under the concrete channel of the Tuck Lateral, through the Northglenn Subdivision Filing 118, and into the Par 3 golf course. F rom the golf course through to the confluence the water course consists of a well-defined open channel, which passes through four ponds and across the major thoroughfares in various-sized culverts. 3. Tributary NE The third largest sub-basin is Tributary NE, which is a left bank tributary to Grange Hall Creek about one-quarter mi Ie above Colorado Blvd. The sub-basin contains 786 acres (1.23 square miles) and extends from the confluence to just south of 120th Avenue at Washington Street. From the confluence to York Street the basin is mostly contained within Thornton, with a small area in Adams County and Northglenn. Above York Street the basin lies entirely within Northglenn. The major features of the basin are the Union Pacific Railroad, York Street, and 112th Avenue. The water course above the Union Pacific Railroad consists of street and storm sewer flow. Below York Street the water course consists of a reasonably well- defined open channel which passes through two small ponds and across the major thoroughfares in various-sized culverts. 4. Tributary N The fourth largest sub-basin is Tributary N, which is a left bank tributary to Grange Hall Creek approximately one-quarter mile below the Union Pacific Railroad. The sub- 8