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316 BULLETIN OF THE ASSOCIATION OF ENGINEERING GEOLOGISTS Figure 9. Thin willows un scarred by debris flow that buried trunks to a depth of 0.35 meter (1.1 ft). South Halfmoon Creek tributary near Leadville, Colo., area 3 on Figure 2. Ruler in center of photograph is 0.15 meter (0.5 ft) long. locities at the edges of the flow. In all these cases damages would have been much greater if the large boulders had been transported by a waterflood, Another phenomenon attributed to the plastic character of debris flows with strength was reported in the local Ouray newspaper, the Ouray Count)' Pla/adealer (July 2, 1973). Cascade Creek drains 3.4 km2 of steep mountain topography before join- ing the Uncompahgre River in a narrow concrete flume through Ouray. In describing a recent "flood," the Plaia dealer reported: "The flume blockage created a phenomenon not often seen, For some time after the flume became plugged all the way from the river to Main Street Bridge. water was running about two feet above ground level in a neat path as if be. tween invisible walls, for twenty feet before it spread out where land leveled off." Our observations indicate that this phenomenon is reasonably explained by channel blockage caused by a debris flow of considerable strength. This is the mechanism by which levees form along debris. flow channels, Levees mantled by coarse boulders along steep mountain channels following large waterfloods sim- ilar to debris.flow levees have been reported (Stew- art and LaMarche, 1967), Matthes (1947) de- scribes a number of forms of macro turbulence that occurs in swift and deep streams. Vortex action, called kolks, similar to tornadoes in air, cause up- ward suction and lift of coarse bed materials, How. ever, the crests of these water-flood levees are as much as I m (3 ft) below flood high-water marks " , , ",,"., Figure 10. Debris-flow levee and undisturbed forest litter, South Halfmoon Creek tributary near Leadville, Colo., area 3 on Figure 2. Levee is more than 1.5 m (5 fr) above undis- lurbed forest litter. on the valley sides, thus helping to distinguish those levees from debris.flow levees. Finally. forest litter and ground cover are dis. turbed beyond channel margins during waterfloods, Undisturbed forest litter occurred outside levees that were 2 m (6,5 ft) high and on ground lower than the bottom of stream channels in the debris.flow areas we visited, Only a viscous debris flow could deposit boulder levees without flowing into adjacent lower ground where forest litter was undisturbed. The debris flow along the East River tributary (Ta- ble I) was restricted to one channel on the debris fan that was bordered by coarse boulder levees that lacked a fine-grained matrix. However, the absence of a fine-grained matrix alone is not sufficient evi- dence to indicate that the 1977 flow was a water. flood, Some fine matrix material was found beneath the surface boulders, and lobes of silty material 10 to 20 mm (0.4-0.8 in) thick were deposited over pine litter along the outside of the levees in the East River tributary area. Observations of older debris. flow levees documents the winnowing of the matrix material with time, It appears that much of the fluid phase of the debris flow (Rodine and Johnson, 1976) drained out of the levees after they formed, These runout zones extending from the base of snouts and levees may be well preserved. The outside edge of a debris-flow levee along the unnamed tributary of South Halfmoon Creek near Leadville, Colo., is shown in Figure 10, Note the undisturbed forest litter to the right even though the ground is more than 1.5 m (5 ft) below the top of the levee. A debris flow of considerable strength was probably required to deposit the boulders on .'