8245 - Laserfiche WebLink

Home Browse Search

This assemblage of geomorphic features, along with that part of the channel upstream from the constriction whose hydraulic characteristics are controlled by the occurrence of the rapid, was referred to as a fan-eddy complex by Schmidt and Rubin (1995). Generally, debris fans are of low gradient in Desolation Canyon but are of steep gradient in Gray Canyon. Debris fans in Desolation Canyon are often very large, sometimes with the distal end spanning over half a kilometer or more (plate 1). Despite their size less than 25 percent of their surface area is active. These relatively small active surfaces deliver the sediment that restricts flow and causes rapids and eddies, while the main fan is often so large that it acts more as a meander bend than a constriction in the channel (plates 2,3 and 4). Gravel bars are abundant in all reaches. Characteristics of the Flow Field at Different Discharges At moderate discharge, constricting debris fans create eddies on their lee side. At lower discharges, eddies typically shrink in size, and eddy sand bars become emergent. The primary eddy return current channel in each eddy becomes inactive and is an area of i low to stagnant velocity. At very high discharges, active portions of debris fans may be l overtopped. When these fans are sufficiently submerged, the downstream eddy no longer exists (plate 4). In canyons with low gradient fans, overtopping occurs at relatively low discharges; overtopping cannot occur where fans are very steep and extend to high l elevation. Generally, the steeper fans of Gray Canyon cause large recirculating eddies to I exist over a larger range of flows than is the case in Desolation Canyon. In the Cedar I Ridge reach the combined area of eddies increased linearly with discharge (figure 4). Availability of Habitats in the Cedar Ridge Study Reach Shorelines were delineated into six major categories; silts and muds, sands, densely,' vegetated fine-grained alluvium, gravel, debris-flow deposits, and talus and bedrock (Tablet 1). Silts and muds generally occurred as terrace-like deposits in areas of low velocity or in areas of downstream flow upstream from rapids. Sand deposits also occurred as channel-I I margin deposits in low velocity downstream flow, but were more common in or near the