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I I - 4 Basin 2. Basin 2 is a geologically complex basin located generally south of Lena Gulch and between Hogback and Green Mountain. In gen- eral, it does not contribute heavily to runoff flows because muc:h more of the basin has natural retention Ot- man-made retention. The type of surface formations range from the sandstone-shale outcroppings of the Hogback to the Piney Creek alluvium which has surficial deposits of clayey silts and sands underlain by older gravelly alluvium. Some of the area is also comprised of the Denver format ion which is an im- pervious claystone. Lena Gulch begins at the tip of this basin at the confluence of Jack- son and Apex Gulches and runs eastward through the trailer court~ of Golden Terrace Village, Tin Cup Vii lage and Ale Trailer Sales. The natural streambed has been replaced by a system of man-made ditches and underground conduits that are eithel. eroded or fi lied We th sedi- ment and debris. In some locations, lal~ge amounts of sediment have been deposited whereas in other areas the channel has eroded verti- cal banks in claystone-type material. The development in these areas docs have a heavy impact on the f;ood flows since they are essentially impervious and create Ilflashll type fIOl'i. A new trailer park development southwest of the 1-70 and U.S. 6 Interchange has and will have more ilT'pact as it nears completion. The planning projections of Jefferson County show that an industrial type area will be located near 1-70 and Rooney Road and it isquite probable that this area also will have considerable impact unless the appropriate care is taken in the development design. Basin 3. Basin 3 also has a wide range of geologic formations v/ith respect to infiltration rates. Much of the basin is porous Piney Creek alluvium and sandy to clayey al'luvium; hO\-Jever, the boundat-y areas of the basin are made of the Denver formation claystone. in- filtration testing gave steady rates of 2.4 inches per hour. This area is to remain largely residential with stri~ commcrcia~ areas along the major roads. Significant rncl~eases in flow could occur if a high density of impervious development were to be constructed. The trai ler court development below U.S. 6 presently increases the streamflow. This development, Mountain Slide Mobile Estates, has also moved Lena Gulch north to a small low capitClI channel. The undercapacity of thi s channel is qui te apparent when compal.ed wi th the State High,'jay. The State Highway box culvel~t, several hundred feet upstream, consi':,ts of two IO-foot wide by 10-foot high culverts whereas the trai lee court trapezoidal channel is only II to 12 feet wide and 3 1/2 to il feel deep.