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<br />and sandstone (Robinson, 1975). This formation interfingers with the Maroon <br /> <br />Formation but produces a small percentage of the debris in the three basins <br /> <br />studied. Small quantities of basalt are also present: on the steep slopes. <br /> <br />As discussed below, an abundance of debris exists in the basins. These <br /> <br />steep, unstable slopes are located in the lower basins below elevations <br /> <br />of approximately 6700 ft (2040 m) (Figure 1). <br />The third necessary condition for debris-flow formation ,. the presence <br /> <br />of clay in the soils. The clay mixes with the debris, and when combined <br /> <br />with the necessary amount of water, gives the flows the strength to transport <br /> <br />the coarser debris and large roc.ks down the channels. In this area sufficient <br /> <br />quantities of clay are produced from weathering of the Maroon Formation. <br /> <br />The fourth condition required for debris-flow occurrence is the development <br /> <br />of the critical ratio of debris to water in the flows. For debri s flows <br /> <br />to occur, it is necessary that the flows contain approximately 20 to 70 <br /> <br />percent water by volume. If a rainfall event produces large volumes of <br /> <br />water relative to debris, water flooding ~rill dominate the transport process <br /> <br />and solid material will be transported as bedload and suspended load. <br /> <br />If, however, the rainfall e'lent causes the production of large volumes <br /> <br />of debris compared to water, debris flows will result. The drainage basins <br /> <br />studied satisfy this condition for debris flowage because adequate volumes <br /> <br />of debris are moved into the channels and transported by runoff waters. <br /> <br />- 8 - <br />