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IV-3 Washington Street which also receive flooding. Downstream of this area the structures are mostly meat packing plants, offices, taverns, fabricating plants, and scrapyards. Much of this land is open space, road and rail- roads. The sewage treatment plant located in this reach is not included in the flooded area. REACH 7: FRANKLIN TO SAND CREEK The Burlington Dam causes a 4 to 5 foot rise in the 100-year flood water surface elevation. This rise is carried upstream of the Franklin Street Bridge. In addition, the York Street drop structure causes a 3 foot rise in water surface elevation. This structure is designed to protect a 60-inch water line which crosses the river at this point. Approximately 250 acres are flooded on the left bank in this reach as the result of water that enters the overbank area upstream in the Interstate 70 area. The land flooded in this reach includes many large open spaces, meat packing plants, scrapyards, construction company offices and a fabricating plant. Of these structures, only 3 businesses are actually flooded. The depths of flooding are from 2 to 3 feet. The floodplain is 2,400 feet wide in this reach and it includes several major streets and railroads. limits of overbank flooding are generally determined by the natural topogra- phy of the floodplain rather than man-made levees or constraints. From Sand Creek to Steele Street the channel is generally naturally graded. This area is dominated by numerous road crossings including 1-270, 1-76 and East 74th Avenue. From Sand Creek to Steele Street there is generally little flooding in the overbank. Overbank flooding does occur between East 74th Avenue and Steele Street and is caused primarily by backwater from the Union Pacific Railroad bridge downstream of Steele Street and the severe meander associ- ated with the bridge. Degradation upstream of the bridge of approximately 4 feet has, to some extent, mitigated the flooding problem. From Sand Creek to 1-76 the channel is confined by levees approximately 400 feet apart. The overbank areas outside of the levees are susceptible to shallow flooding during the 100-year event. Between 1-76 and East 74th Avenue the overbank areas are mined and the levees are, for the most part, ineffective. Clear Creek enters the South Platte River 500 feet downstream of 1-75. REACH 8: SAND CREEK TO BASELINE ROAD The reach of the South Platte River between Baseline Road and Sand Creek experiences flooding due primarily to insufficient channel capacity. Cross- ings of less than lOO-year capacity also contribute to some minor degree to the flooding situation in this reach. Levees exist in much of this reach to protect agricultural land and on-going and past gravel mining operations from flood flows, but they are largely ineffective in containing flood events within the channel for flows in excess of about 6,000 cfs. From Steele Street to East 88th Avenue the river is confined by levees on the east side and natural topographic features on the left. Tho City of Thornton controls the bulk of the floodplain land and has municipal water storage facilities outside the levees on the east side. Downstream of the storage facilities the 100-year flood flows overtop the levees and over 12,000 cfs leaves the channel and flows overland to 88th Street. These flows re-enter the main body of flow downstream of 88th Street. Overtopping may be due in part to 3 feet of channel aggradation upstream of East 88th Street resulting from the construction of a 4 foot drop. The 100-year floodplain in this reach can best be described as uncontrolled by the upstream dams. The floodplain width is generally between 4,000 and 5,000 feet and many agricultural and gravel mining lands are flooded. The From East 88th Avenue to Baseline Road the overbank flooding is due primar- ily to insufficient channel capacity. The East 104th Avenue and East 160th Avenue bridges are of less than 100-year capacity, but they do not have a large impact on the extent of overbank flooding. In general, this reach has degraded approximately 4 feet with the exception of the river immediately upstream of the Brantner Ditch diversion dam which has aggraded 2 feet.