Arc Hydro will not calculate flow rates in cubic feet per second (cfs). Arc Hydro is used to help the parameters that go in to equations or models that are used to calculate the flow rate.
The tools in Arc Hydro are made to help automate the determination of the the parameters that come from watershed delineation process such as area parameter, flow path length and slope, watershed centroid, using the watershe polygons to convert raster based land values of impervious surface or infiltration rates to averages for the watersheds; thing like that.
Hope this helps,
Mark
