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The floodplain on Tributary "C" above Lowell Boulevard varies from 50 to 350 feet in width for most of the reach. It then widens to 800 feet in the backwater area just upstream of Lowell Boulevard. Below Lowell it narrows to 100 feet in width to its confluence with Tributary B. Upstream of 1-25, the combination of the broad, flat topography, the elevated road crossing and the lack of any undercrossing of the interstate highway, creates a significant backwater effect. After ponding to an elevation of 5175.1 feet, the 100-year peak discharge will overtop the highway about 2300 feet south of West l44th Avenue. On the east side of 1-25, the flows enter a fairly well defined drainage course that carries runoff to the east towards Big Dry Creek. Floodplain widths vary from 270 feet to 1000 feet, with the greatest widths occuring just upstream of Washington Street, where greenhouse structures have been constructed immediately adjacent to the McKay Lake Basin drainageway. All of the greenhouse buildings situated just west of Washington Street are located within the combined Big Dry Creek - McKay Lake Basin floodplain. I I I I I I I I I I I I I I I I I I I channel. The 100-year floodplain covers the channel and the adjacent 136th Avenue right-of-way. A floodway was defined for the shallow flooding area to the south and is shown on Pl an Sheets Band 9. For pl ann i ng purposes, the fl oodway shown is that area required for the channel and appropriate right-of-way to convey the 100-year flow of 1190 cfs to Big Dry Creek. equa 1 s 5246.1 feet. After overfl owi ng the eastern Me KdY La ke reservo i r emban kment, flood flows pass through relatively broad, open agricultural fields. From McKay Lake down to Interstate 25, the floodplain is fairly wide, ranging from 350 feet to 850 feet in width. The Tributary "A" floodplain begins downstream from Zuni Street at the outlet from the Westlake Subdivision detention pond. The floodplain is shallow overland flow 200 to 350 feet wide and less than one half foot deep up to the point at which it ties into the Quail Creek floodplain. Due to the relatively flat topography and absence of any well defined drainageway, about 280 cfs of the total 1180 cfs 100-year flood peak separates from the main flow between station 79+00 and 85+00. This 280 cfs flows generally southeast and remains separate from the main drainageway until reaching the backwater area west of 1-25 created by the roadway embankment. The Tributary "B" floodplain varies from 80 to 250 feet wide just upstream of the l44th Avenue-Lowell Boulevard intersection. The depth of flow over the road is approximately 2 feet. Downstream of the intersection the floodplain widens to 1150 feet where a farm pond is inundated. The house and several other structures are in the 100-year floodplain below the pond embankment. The floodplain then narrows to 500 feet in width for the remainder of the reach. The confluence with Quail Creek is within the irrigation reservoir on Quail Creek. The Tributary "D" floodplain is 270 feet wide at the upstream study 1 imit just below l24th Street. The channel becomes more well defined proceeding downstream and the floodplain narrows to 60 feet just above the confluence with Quail Creek above Equity Ditch. McKay Lake Basin North of West l44th Avenue, the McKay Lake Basin floodplain is contained in a relatively well defined drainageway. Upon crossing West 144th Avenue, however, the McKay Lake flood flows pass into the McKay Lake reservoir pool. Floodplain limits at the McKay Lake impoundment were determined assuming all flood flows would remain within the basin, overtop the eastern reservoir embankment and flow to the east. The backwater elevation in McKay Lake based on this assumption 20