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~' MIDPROJECT PROGRESS REPORT October 30, 1989 TITLE: PALEOHYDROLOGIC FLOOD INVESTIGATION OF ARTHURS ROCK GULCH AND VICINITY NEAR HORSETOOTH RESERVOIR, FORT COLLINS, COLORADO COOPERATING AGENCY: U.S. Bureau of Reclamation PRINCIPAL INVESTIGATORS: Robert D. Jarrett and Christopher F. Waythomas PROJECT DURATION: March 1989 through May 1990. BACKGROUND: Paleohydrology in the Colorado Front Range is important because short-term hydrologic records are insufficient to assess long-term hydrologic variability. To resolve the problem. methods for extending existing hydrologic records over long time scales are needed. A relatively new approach, that complements engineering hydrology, involves the application of paleohydrology to determine regional scale hydrologic variability over long time intervals (0.01 to 10 ka). Paleohydrology is the study of the movement of water and sediment in channels before the time of continuous records of direct measurement. Paleohydrologic data can be derived from stratigraphic based studies of fluvial deposits and features. A paleohydrologic investigation typically involves studies of the hydrologic, hydraulic, botanical, sedimentologic, stratigraphic, and chemical information preserved in former or present river channels, flood plains, and terraces. New paleohydrologic techniques were recently developed by R.D. Jarrett and J.E. Costa of the U.S. Geological Survey to provide important additional information on hydrometeorology and to document the magnitude and frequency of floods in Colorado. Use of long-term hydrologic records helps reduce the uncertainty in hydrologic analysis associated with the design of hydraulic structures and for flood-plain management. OBJECTIVES: Preliminary investigations of flood deposits in Arthurs Rock Gulch indicated one or more large floods have occurred that may represent one of the largest floods in the Colorado Front Range. The focus of the study is to conduct research in Arthurs Rock Gulch to construct the paleoflood history of the basin. The emphasis at present is Arthurs Rock Gulch and includes a reconnaissance of other tributaries that drain into Horsetooth Reservoir. The three objectives of the study are to: 1. Conduct paleoflood investigations in Arthur's Rock Gulch and to locate flood deposits in nearby drainage basins to estimate the magnitude of floods in Arthurs Rock Gulch. 2. Estimate the approximate date of the floods. 3. Estimate the frequency of the floods. The present study will provide valuable information on floods that have occurred in Arthurs Rock Gulch during the last several thousand years. The results of this study will be integrated into existing hydrologic data from past studies to provide an improved understanding of the magnitude and frequency of extraordinary flooding in Arthurs Rock Gulch and the Colorado