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I 5 I I Gregory Creek was historically a "mountain stream" except for several hundred feet near its confluence with Boulder Creek. There a small alluvial fan was created, The historical channel has been disrupted by home building below Flagstaff Road. As a result, the natural flood plain character has been considerably altered and widened, Below 7th Street, the encroachment of houses in the widening canyon has created a severe flooding hazard. I I , I I I II I I I I I I I I I I I Bluebell Canyon Creek and King Gulch are rigid and well defined until they enter the residential areas. Bluebell Canyon Creek flows through Chautauqua Park into a natural channel for two blocks and then into a small man-made channel. This area is the top of the alluvial "fan" as evidenced by the large boulders deposited in Chautauqua Park. The man- made channel flows into the Anderson Extension Ditch which crosses into other basins. Flood waters are carried into Skunk Creek and into other basins by the Anderson Extension Ditch in an unplanned and haphazard manne r. King Gulch retains its mountain character until 15th Street where it has been directed in a man-made channel along the King Avenue alignment. At 17th Street the water is received by a small storm sewer line which out- falls into Skunk Creek. Flood waters overflow in the Anderson Extension Ditch and Skunk Creek along with the Bluebell Canyon Creek Overflow. Skunk Creek and Bear Creek have much the same character with well defined channels of considerable length flowing through mountainous terrain, The channels historically meandered down an alluvial fan, recrossing and build- ing the alluvial fan. As urbanization occurred, the streams were inter- cepted at a convenient location several "undred yards below the start of the alluvial fan. Thereupon the streams were guided through the urban area and alignment set by the available spaces left after urbanization by landowners and/or by the path left after a flood, Both streams flow into the Wellman (Empsom) Ditch which delivers water to the Public Service Company Valmont Plant. Viele Channel has the same general hydraulic and geological character as Bear Canyon and Skunk Creeks except that a reservoir has been constructed near the top of the Urban Basin. Below the reservoir a basically adequate channel has been developed through the residential area near Broadway. The area downstream of 46th Street is flooded as it has been historically, The Boulder Turnpike enlarges the flood plain considerably. David's Draw has a small tributary area from the foothi lIs, The water flows from the greenbelt land into the South Boulder Creek flood plain along a naturally flat thalweg which parallels South Boulder Creek. Much of the runoff from David's Draw infiltrates into the alluvial acquifer. PAST FLOODS Documentation of past flooding problems within South Boulder has been less than for North Boulder. UrbanizatIon has taken place rapidly in recent years in South Boulder and the natural drainageways have been encroached upon.