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A computer application and national geospatial database have been developed to support the calculation of flooding flow (Qf) and threshold runoff across the conterminous United States and Alaska. Flooding flow is the flow required to cause a stream to slightly overflow its bank and cause damage. Threshold runoff [L], defined as the depth of runoff required to cause flooding, is computed as flooding flow divided by the unit hydrograph peak flow. A key assumption in this work is that the two-year return flood (Q2) is a useful surrogate for flooding flow. The application described here computes flood magnitude estimates for selected return periods (Q2, Q5, Q10, etc.) using regression equations published by the U.S. Geological Survey for each of 210 hydrologic regions. The application delineates basin boundaries and computes all basin parameters required for the flood frequency calculations. The geographic information system database that supports these calculations contains terrain data [digital elevation models (DEMs) and DEM derivatives], reference data, and 89 additional data layers related to climate, soils, geology, and land use. Initial results indicate that there are some practical limitations associated with using Q2 regression equations to estimate flooding flow.