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.. 1995: Where the Past (Paleoflood Hydrology) Meets the Present, Understanding Maximum Flooding Robert D Jarrett, US Geological Survey, Box 25046, MS 412, Denver, Colorado 80225 303-236-6447; fax: 303-236-5034; internet: rjarrett@usgs.gov Paleoflood hydrology has become accepted in the scientific, engineering, and regulatory communities and increasingly is being used to complement engineering hydrology throughout the Rocky Mountains and in other western states. Uses include design of flood-warning systems, flood-plain management, design of hydraulic structures in flood plains. management of riparian ecosystems, and better utilization of the Nation's water resources. One of the greatest benefits of paleoflood hydrology are estimates of maximum flooding in many thousands of years. However, one long standing issue pertains to the accuracy of discharge estimates made from paleostage indicators (PSis). The most commonly used PSI in paleoflood studies of higher-gradient streams (say gradients larger than 0.001 m/m) are flood bars, which are comprised of sands, gravels, and boulders. These bars are preserved for many tens of thousands of years until larger floods produce larger bars. Typically, the elevation of the top of the flood bar is used as the minimum elevation of the flood that deposited the bar. Hydrologic condition during 1995 allowed for an evaluation of the relation between PSis and 1995 high-water marks (HWMs). Colorado had one of the driest winters in decades through March 1995 with snowpack in many basins only 50 percent of normal. However, record precipitation occurred from March through June and mostly as snow, which resulted in a record snowpack in many basins in Colorado. Near to record runoff occurred in many streams during June and July, which mobilized and transported large quantities and sizes of bed material. These sediments were deposited as bars in the channel and on the flood plain; many bars were observed forming during peak-flow conditions. The bars contain sand through boulder-sized material. Surveys of these bars, HWMs, and channel geometry were made for more than 25 streams that have a wide range of widths, depths, slopes, and particle sizes. In all but the steepest streams, the elevation of the top of the bar (PSI) essentially is the same as the HWMs. In streams having gradients larger than about 0.04 m/m, the top of the bars protruded above the HWMs. Thus, the elevation of flood bars preserved in channels and on flood plains are reliable indicators of the maximum flood stage and thus discharge. These results also provide valuable insight on sediment-transport processes, which are complex and poorly understood in mountain rivers.