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e . . consists of alluvium largely composed of fine sands, but also with areas of cobble or exposed mudstone/sandstone bedrock. Due to the high infiltration rate of the native material, most of the Sand Creek drainage flowed underground as part of the alluvial aquifer. The creek was largely channelized through the airport property as development occurred. In addition, a backwater area developed upstream from the Havana Street crossing originally consisting of a series of 36-inch diameter culverts. As development continued upstream of Havana Street, the culverts became undersized, causing backwater flooding in the Bluff Lake site. This flooding, along with seepage from the dam, contributed to the development of wetlands largely composed of non-native species. Alter the crossing had been washed-out a number of times, It was improved in 1987 with the replacement of the culverts by a clear-span bridge with a much larger conveyance capacity. Sand Creek has experienced a metamorphosis with urbanization of its upper basin. The creek has developed a baseflow of 30 to 40 cubic feet per second (cfs) and a higher recurrence of flood flows resulting primarily from summer thunderstorms. As a result, there was evidence of bed and bank degradation intermingled with depositional areas as the channel attempted to conform to changing watershed characteristics and reestablish a dynamic stability. Although grade control structures have been built upstream and downstream of the Bluff Lake site, the channel gradient through the site was, prior to improvements, 0.0044 ftllt. The channel through the site had become deeply incised exhibiting 10 to 15 foot tall cutbanks, with a width to depth ratio through the site averaging about 3 to 5. Sinuosity was also low, in the 1.1 range. The cutbanks were a result of the channel attempting to increase its width to depth ratio and meander pattern. Channel downcutting was also evident on the east property line where a treated wastewater effluent line crossing of the channel had been undercut by 3 feet. The Sand Creek Major Drainageway Planning Study conducted by the Urban Drainage and Flood Control District, Commerce City, Denver, and Aurora, recommended the following improvements: . Construction of a levee along the north overbank next to the County Jail . A 7.foot drop structure a the east property boundary . A new bridge at Havana Street with a 250-foot span with roadway overflow sections . Maintenance access The master plan intended to allow the channel to degrade naturally about 2 to 211, feet with a projected stable gradient of 0.22 percent controlled by downstream improvements. With the construction of the Havana Street bridge in 1987, the rate of channel degradation predicted by the master plan began to increase with the result that a major meander approximately 2,000 feet upstream was cutoff, further increasing the gradient through the site. This change in morphology, along with increased development of the upstream watershed, accelerated channel and bank erosion in the 1990s. Bluff Lake Improvements Both Camp Dresser and McKee Inc. (COM) and Aquatic and Wetland Company (AWC) were named in the Consent Decree to provide design and construction services. This was done because the Sierra Club had worked on a similar project with AWC on Clear Creek and wanted them to design the low flow channel and build the channel and wetland improvements. COM had worked with the Department of Aviation to design improvements to capture and treat ethylene glycol contaminated stormwater; COM also completed the Airport's Phase I Environmental Assessment. It was decided, because of the Consent Decree conditions, that the contract would be executed as a design/build project with COM's subsidiary, COM Engineers & Constructors Inc. (COM E&C), as the prime contractor, and AWC as a subcontractor providing channel and wetland construction services, as well as design services pertaining to biotechnical bank treatments and wetiands enhancement.