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11-6 Basin 5. The portion of Green Mountain that drains to BasIn 3 and Lena Gulch above 1-70 forms Basin 5. This area also has an extreme variety of geological fornations and the highest lands'l ide hazards. Green Mountain Itself has a conglomerate formation of pebbles and cementous material that 15 more permeable than the other mountainous formations. The slopes are formed of el the," al,. luvlum or claystone-type materials that have both a high swell and I ands II de potent I a I. Several thalwegs exist that are generally protected by heavy vegeta- tIve cover. These streams JoIn together Into one stream that crosses 1-70 near the Colfax Interchange and joins with Lena Gulch Just above Isabell. Infiltration testing near Jefferson County Fairgrounds conducted in both relatively natural and man-altered areas Indicated moderate In- filtration rates of 1.3 Inches per hour. This area and Its streams are very sensitive to development. Impervious areas 1 Inked with a developed stream system will create high peak discharges. The stream that draIns this area Is a major dralnageway and should be protected from further channel restrictions. Much of the upper Green Mountain area Is planned and zoned as park area. However, most of the other area Is slated for deve'lopme,nt. This development should be planned with detention storag!' and flow control devIces to prevent increases In flow and erosIon "ates. Be,cause of the nature of the soIls In the area, It wIll be necessary to provIde more right of way than Is usually necessary near detentIon facilitIes and swales to prevent erosion and slides. Basin 6. This basin Is the area below 1..70 that draIns into Lena Gulch above Maple Grove Dam. The soils of the area are primarily alluvial with moderate Inflltr8tlon rates that average 1.3 Inches per hour. As the result of the unearthing and mixing of tighter, more expansive subsoils Into the upper layers, lawn surfaces tend to be more Imper- meable than natural areas. The lower two-thirds of this basin Is developed In residential dwellings, with the upper third slated to be developed for both commercial and residential usage. The channel condition In this reach Is unstable, with the channel cutting at a rate of 2 to 4 feet every few years and with heavy sIltation of culverts. The channel drops about 3 feet downstream of the 1-70 twin (2) 12 foot wide by 10 foot high box culverts, where a rock layer of silty sandstone has been revealed by erosIon. The Agricultural Ditch crosses Lena Gulch In a concrete bridge crossing that Is being undermIned. ,Just below the dItch crossbg,