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constructed by the U.S. Soil Conservation Service within the past six years and are shown in Figure 2. These reservoirs significantly reduce the 100-year peak flood flow on Boxelder Creek. The 50-year and lOO--year peak flood flows on Boxelder Creek above the Larimer-Weld Canal with upstream detention have been estimated to be 2780 CFS and 3840 CFS respectively (Reference 1). The main stem of Boxelder Creek approximately 1.5 miles north of I,arimer County Hoad 50 has a maximum flow capacity on-the_order of600CFS. Storm runoff on Boxelder Creek in excess of 600 CFS spills to the southwest towards Interstate Highway 25. This spill was first identified in the draft flood hazard area delineation report for Boxelder Creek (Reference 2). Under natural conditions, piior to any improvements by man, storm runoff from this spill probably spread out over a wide area and flowed from north to south. Natura.I drainage patterns affecting the vicinity of the planned interchange have been altered by the construction of highway and irrigation facilities. Presently during the lOQ-year storm event, approximately 3320 CFS would spill from Boxelder Creek and flow to the south into the northeast quadrant of the planned interchange. There are four concrete box culverts located under the north bound lanes of Interstate Highway 25: three 10-10X4 double concrete box culverts and a 13-16-13X4 triple concrete box culvert. These culverts drain to the west to the Interstate Highway 25 median. 'ro enter these culverts storm runoff must first overtop and flow across the existing frontage road located immediately east of Interstate Highway 25. Storm runoff greater than thE! flow capacity of these culverts would be diverted southward via the existing frontage road and the area to the east. / ~~: There is a ridge between the area along thE~ east side of Interstate Highway 25 and the main stem of Boxelder Creek as shown in Figure 3. This ridge ranges from approximately 2 to 12 feet above the main stem of Boxelder Creek. At Larimer County Road 50 storm runoff from the Boxelder Creek Spill could, under existing conditions, flow to the south .un.de..J:: the Larimer County Road 50 overps.ss, or QY~.J:: Larimer County Road 50 to the south via the roadway sag vertical curve located east of 1-25, or could flow to the west by overtopping southbound 1-25. The hydraulics of these various flow mechanisms, paths and splits <lre complex. The U.S. Army Corps of Engineers HEC.2 watE~r surface profile computer program was used to investigate the relative magnitudes of these flow splits for the ant:icipated 100-year spill from Boxelder Creek of 3320 CFS. Two separate profiles were run. One profile for the Int:erstate Highway possible overtopping of the south bound lanes of Interstate Highway 25 and the other profilE! for the area east of Interstate Highway 25 including thE! frontage road 4