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<br />OCT-30-S6 WED 14:04 <br /> <br />BRR BLD 53 DFC <br /> <br />FAX NO. 2365034 <br /> <br />p, 08 <br /> <br />rainstorm. <br /> <br />East Fork Little Bear Creek near Porcupine ReservoIr <br /> <br />~\y <br />On September 25~on Jensen and I visited the East Fork Little Bear Creek watershed to conduct <br />a reconnalaaance paleoflood stuely for Porcupine Reservoir. Porcupine Reservoir Is located <br />about 4 milas aast of Avon, Utah, at an elevation of approximately 5,400 f.at Paleoflood <br />astimatas wars mada for Cinnamon Creek about 300 feet upstream from LiWe Bear Creek, Little <br />Bear Creek about 300 faat upstream from Cinnamon Creek, and East Fork Little Baar Craek <br />upstream from Porcupine Creek Reservoir at the U.S. Geological Survey streamflowllaglng <br />station. The top of bouldery flood bars, non-inundated surface (with extensive soil <br />development), woody debris. and erosion scars were used to estimate paleoflood depth. Tha <br />woody debris and erosion scars appear to be about 10 to 15 years old (probably corrnponding <br />to a substantial snowmelt runoff) and slightly lower than tha top of flood bars and old, non- <br />inundated surfaces. The flood bar. and non-Inundated surfaces are estimated to be at least <br />sevaral hundred years old (based on the amount of organic. in the '011) and may reflect <br />maximum flood depth since glaciation. The paleof1ood estimates ara summarized in table 1. The <br />maximum paleoflood was estimated to be about 3,500 I13Is for the East Fork Little Bear Creek <br />at the gage (site 3). The peak discharge ranged from 1,100 to 1,400 fl3/s lor the PSis, which I <br />estimated to be about 10 to 15 years old. Most of the trIbutarie. to the East Fork LiWe Bear <br />Creek and to Porcupine Reservoir were visited, but time did not allow us 10 estimate peal< <br />discharges. None of the tributaries had any substantial evidanca of paleofloocla. Nor were thers <br />any substantialllood deposits at or above the September 25th reservoir water level, which was <br />relativ.Ylow (perhapa 30 to 40 feet of drawdown). Had substantial flooding occurred in any <br />^ <br />tributary to the reservoir, a delta likely would have been deposlled since construction of the <br />reservoir (year?). Severa/leet of sand and silt that have accumulated on parts of the exposed <br />reservoir bottom and probably correspond to sediments transported primarily by snowmelt <br />runoff (as inferred from flood-deposited sediment structure). Sedimentologic analysis would <br />confirm the total sedimentation amounts, likely tributary source areas, and magnitude/duration <br />01 flood events. <br /> <br />7 <br />