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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />case (B), a major flood on the tributary stream carries material down from <br /> <br />the upper reaches of its drainage basin, remobilizes the old debris fan <br /> <br />deposit and leaves a new deposit on the debris fan. The tributary stream <br /> <br />usually occupies a new channel and everything on the debris fan prior to <br /> <br />flooding is modified or destroyed in the process. In the case of a main- <br /> <br />stream flood with little or no flooding of the tributary stream (C), that <br /> <br />part of the debris fan that lies within the mainstream flood plain is com- <br /> <br />monly eroded away by the mainstream flood. In the last case (D), simultane- <br /> <br />ous flooding of both the main and tributary streams results in modification <br /> <br />of both the mainstream channel and the debris fan. At some locations during <br /> <br />the flooding of the Big Thompson River, the competence of the mainstream <br /> <br />was sufficient to carry away essentially all of the new debris deposit on <br /> <br />the fan. In the case of tributary streams that were of sufficient competence <br /> <br />to carryall their bedload, this situation is offered as a plausible ex- <br /> <br />planation for the absence of debris deposits at several stream confluences <br /> <br />where one could have reasonably expected debris fans to form. <br /> <br /> <br />33 <br />