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<br />- 17 - <br /> <br />topographic map (plate 3) shows a dozen or more temporary distributary <br /> <br />channels in the area between th~ present Slate Creek channel and the <br /> <br />one that was occupied before th~ diversion. Several of the smaller <br /> <br />distributary channels terminate in a lobe indicating that the final <br /> <br />event consisted of mud that was viscous enough to pile up and obstruct <br /> <br />the channel rather than continu~ flowing. <br /> <br />Because of the condit:l.ons and history above described, we <br /> <br />consider it highly probable that future erosion, landsliding, mudflow, <br /> <br />or other natural alterations of the existing channel will continue to <br /> <br />bring about shifting of floods among the various older channels. <br /> <br />Shifting of distributary channels is the normal cour,s.EC of..events on <br /> <br />alluvial or mudflow fans. However, the exact timing, location, or. <br /> <br />nature of the event which brings about the channel change is not pre- <br /> <br /> <br />dictable. Because of the high degree of unpredictability, and the <br /> <br /> <br />high energy and large sediment volume of alpine-type mudflows, we <br /> <br /> <br />feel that control structures with any possibility of success would be <br /> <br />expensive to build and maintain, and that their long-range effective- <br /> <br />ness would be problematical. <br /> <br />Nature of Mudflows in the Marble Area <br /> <br />Because they are usually short-lived events and are seldom <br /> <br />.observed in their entirety even by geologists and hydrologists who <br /> <br />are called upon. to interpret and control them, we believe there is a <br /> <br />tendency to underestimate the seriousness of mudflow phenomena and <br /> <br />the difficulty of effectively controlling them. In order to ade- <br />quately convey the actual nature of alpine-type mudflows, We will cite <br /> <br />some eyewitness accounts, and describe our opinion as to how the flows <br /> <br />form, become channeled, and spread out over the lower slopes with dis- <br />