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From evaluation of topographic mapping, the historic drainage course, in the McKay Lake watershed appears to pass through the existing reservoir site to the southeast, running roughly parallel to l44th Avenue. A field survey of the two reservoir embankments, however, indicates that the crest of the southern embankment is considerably lower than the southeastern embankment. Consequently, the McKay Lake impoundment will effectively cause a transbasin diversion of floodwaters from the McKay Lake watershed into the Quail Creek basin. All runoff in the McKay Lake watershed upstream of McKay Lake passes through the reservoir. Hydrologic analysis indicates that the magnitude of the 100-year flood peak at McKay Lake equals 1180 cfs. Under existing conditions, virtually all of this inflow to McKay Lake will overtop the southern embankment and pass into Quail Creek. Routing the 100-year flood hydrograph through McKay lake, assuming a full reservoir condition, causes the 1180 cfs peak discharge to be attenuated to 275 cfs. This is the magnitude of the potential overflow from McKay Lake to Quail Creek under existing conditions, assuming that the embankment would not fail. 1-25 is the fact that the basin divide between Quail Creek and McKay Lake is not well defined. The Quail Creek channel is perched relative to the adjoining McKay lake topography. When flows go out-of-bank on Quail Creek between Huron Street and 1-25, they will flow down-grade into the McKay lake basin. Flows can also transfer relatively easily between the basins through a local drainage ditch which runs parallel to the highway. A 32 inch high concrete median barrier situated between the north and southbound lanes of 1-35 further increases the backwater effect created upstream of the highway. I I I I I I I I I I I I I I I I I I I unique considerations for floodplain delineation. In particular, the McKay Lake impoundment is situated adjacent to the McKay Lake - Quail Creek basin divide, with the southern reservoir embankment defining the basin divide. The approach taken in this study was to not give credit for the effect of the concrete median barrier on the distribution or direction of flow in a downstream direction, however, the backwater effects of the barrier were considered in delineation of the upstream floodplain. All flows were kept within their respec- tive basins for downstream floodplain analysis. Combination of flows from the McKay lake and Quail Creek basins upstream of 1-25 was not considered. For purposes of downstream floodplain delineation in the McKay lake basin, all flow was assumed to remain within the basin. Similarly, the potential overflow of 275 cfs from McKay Lake into Quail Creek was not included in the Quail Creek flood flows, however, the area of shallow flooding between McKay Lake and Quail Creek caused by the 275 cfs of potential overflow was delineated. In the McKay lake basin, flows will cross 1-25 at a low point on the road profile, approximately 2300 feet south of west 144th Avenue. In addition, there is a break in the median barrier at a point about 3500 feet south of West 144th Avenue, allowing for a portion of the McKay lake flow to Cross 1-25 at this location. Interstate 25 In the Quail Creek basin a high paint of ground on the east side of the highway causes the flow to split and move either northeast into the Quail Creek channel or generally south along the Interstate 25 roadway and overland to the Big Dry Creek channel. It was calculated that 2500 cfs of the total 3730 cfs peak discharge on Quail Creek will pass into the existing drainageway along West l36th Avenue. The remaining 1230 cfs passes to the south in roadside ditches along the highway and as overland shallow flooding to the southeast to Big Dry Creek. Interstate Highway 25 crosses the lower end of the Quail Creek and McKay Lake basins, perpendicular to the drainageways in each basin. The 1-25 roadway embankment is elevated above the upstream ground surface, causing detention of runoff. Upstream of 1-25 the topography in the McKay Lake basin is gently sloping and a considerable volume of water (in excess of 200 acre-feet) could be impounded prior to overtopping of the highway and the concrete median barriers. There are presently no improved storm drainage crossings beneath 1-25 in the McKay Lake basin. Another feature relating to floodplain delineation upstream of 18