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<br />Paleoflood data <br /> <br />The frequency of extraordinary floods can be estimated in a number of <br /> <br /> <br />ways (Costa, 1978a, 1978b). In the Big Thompson River downstream from <br /> <br /> <br />Estes Park following the catastrophic flood during 1976 (McCain et al., <br /> <br /> <br />1979), radiocarbon dating of truncated and eroded landforms yielded an <br /> <br /> <br />estimate of the minimum length of time since an event of similar magnitude <br /> <br /> <br />had occurred in the valley. Radiocarbon dating of older boulder deposits <br /> <br /> <br />from earlier floods preserved in river terraces and exposed by erosion <br /> <br />following the 1976 flood also provided evidence of the length of time since <br />a flood of similar magnitude occurred. <br /> <br />In the lower Big Thompson River basin, three radiocarbon-dated <br /> <br /> <br />alluvial fans were used to indicate the rare occurrence of floods like the <br /> <br /> <br />one during 1976. The 1976 flood eroded fans that essentially were <br /> <br />undisturbed for 6,600 to 10,400 years. The flood also eroded old river <br /> <br /> <br />terraces and exposed some very coarse older flood deposits in one location <br /> <br /> <br />as shown in Figure 6. These are the largest pre-1976 flood sediments known <br /> <br /> <br />in the valley. A radiocarbon date from the fine-grained deposit on top of <br />the coarse boulders was 10,500 years, which strongly indicates that the <br /> <br /> <br />flood boulders are glacial outwash and were deposited by large floods <br /> <br /> <br />during glacial melting. This evidence indicates that the flood in the <br /> <br /> <br />lower Big Thompson River basin during 1976 was the largest since glacial <br /> <br /> <br />melting, or during the last 8,000 to 10,000 years. <br /> <br />~~ <br /> <br />, <br />