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II -7 the Gulch has eroded badly enough to cause the collapse of a pri- vate residentIal brIdge. The channel has quite steep, man-made and quasi-natural banks that cross layers of clayey and silty sandy alluvium and claystone-like material. A very tight alluvium or colluvIum sample of sandy, si 1 ty clay ""'lS taken from the st,-eambed where It was quite evident that erosion had occurred, exposing this material. Some erosIon control measures have been installed d10ng the low( r reaches of the basIn. The channel capacl ty, ho>~ever, Is somewhat Insuffic- Ient. Below Alkire Street and above Youngfield Street, the channel conditIons fluctuate. Some areas have fair channel capacity and condition. Other locations exhibIt erosIon problems. The culvert under Youngfleld Street was heavily silted, but would probably self clean due to the high head over the culvert. From Youngfleld to Maple Grove Reservoir, Lena Gulch has fair channel capacity with erosion control being the most serious problem as several homes are quite near the stream. Basin 7. This basin Is the area of South Table Mountain that drains into Lena Gulch below 1-70 and Maple Grove Reservoir. The basin has the same general geological condition as BasIn 4 but sheds run- off to Lena Gulch In a multitude of outfall. such as 20th Avenue, Rocky Mountain Ditch, 32nd Avenue, and various hlghw'ay culverts. Much of this area Is planned for residential uses which could create erosion and flow control problems. Careful detention meas- ures should be taken to safely route the flow from this basin to Lena Gulch. BasIn 8. This basin represents the por'tion of the ,,'atershed that Is tributary to Lena Gulch below Maple Grove ReservoIr and above the general locatIon where the carrying capacity Is considerably dimin- ished and widespread flooding occurs. The surface soils are generally colluvii~l wi th the infiltration rates being about two thIrds of an Inch per rour. This rate Is compara- tively low, but reasonable due to the higher percentage of lawn and developed areas. The channel generally has good capacety wIth the exception of under- sized culverts and a few locations where erosion is heavy. Dur':ng a field trip of September 3. 1973, a h~~vy discharge of black water having the appearance of filter backwash from the Maple Grove Fi I tee Plant was noted. This practIce should be stopped.