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<br />, <br />.. .--.,.' <br />,r <br /> <br />overbank was estimated to be about 5 feet per second. There was no flow <br />on the right overbank at this location. Hence, the total flood discharge <br />was estimated to be about 330 cubic feet per second in an expanding, <br />depositional reach where there are flood deposits. The stratigraphy and <br />morphology of the flood deposits indicate they were deposited from <br />neither a debris flow nor a clear water flow, rather they appear have <br />characteristics of hyperconcentrated flow deposits. Hence, the <br />reconstructed discharge above is the discharge of water and sediment. <br /> <br />The nondepositional reaches are relatively steep, straight, and uniform <br />and are located downstream from the depositional reaches. Vegetation <br />(primarily willows) on the channel banks is stripped of bark to a depth <br />of about 2 1/2 feet above the thalweg. Flood deposits in these reaches <br />are generally sand and gravel on the main channel margins and boulders <br />only in the channel. There are no deposits on the overbank. An estimate <br />of the flood discharge was made at an elevation of 9,600 feet. The <br />average channel width is 14 feet and the average flood depth is about 2 <br />feet, based on the height of the flood deposits and scarred vegetation. <br />The channel gradient is about 3 percent. Assuming a flood velocity of 7 <br />feet per second, the flood discharge was estimated to be 196 cubic feet <br />per second, <br /> <br />AGE OF FLOOD DEPOSITS <br />The flood deposits are relatively fresh with little new vegetation <br />growing on the depositional surfaces. Samples of the largest vegetal <br />growth were taken on one of the largest flood deposits. A 1/2 inch <br />diameter, 3 foot tall willow had 5 annual growth rings. A 1/4 inch <br />diameter, 18 inch tall spruce had 4 annual growth rings. These ages <br />suggest a minimum age of 5 years for the flood. Possibly these flood <br />deposits could be related to the high snowmelt runoff year of 1983 (the <br />highest or second highest peak flow of record at nearby streamflow-gaging <br />stations). Alternately, these deposits could have resulted from an <br />intense rainstorm in the basin during about the last 10 years, from a <br />failure of a beaver dames), or failure of a snow and ice dam. <br /> <br />DISCUSSION <br />These flood deposits are related to one of the largest floods in this <br />basin. I have conducted extensive interdisciplinary studies to locate <br />evidence of large rainstorm floods above 7,500 feet. These are the only <br />flood deposits I have seen above 7,500 feet. The unit discharge for this <br />flood ranges from 60 to 37 cubic feet per second per square mile. This <br />compares with a known (based on a recent compilation of all flood data in <br />Colorado) maximum unit discharge of 100 cubic feet per second per square <br />mile above 7,500 feet in Colorado. Below 7,500 feet in Colorado, unit <br />discharges have, on many occasions, exceeded 1,000 cubic feet per second <br />per square mile. Therefore, it does not appear that a large rainfall- <br />flood occurred in the Jim Creek basin. I believe the flood deposits are <br />unique to this basin. The easily erodible debris-flow deposits were the <br />source material for the deposits, rather than having a large flood erode <br />the banks. Also, there are no other older flood deposits in Jim Creek. <br />The flood deposits and channel geometry suggest that this flood <br /> <br />..,:":'-.., <br /> <br />'-,'.'- ",' <br /> <br />;",">' <br /> <br />-" "-""-.'- ~>< <br />