River basins may have gauged and ungauged catchments. To determine flow contributions from the catchments distributed parameter modeling is required. Geographic Information System (GIS) can handle spatially varied parameters efficiently and is useful in developing IUH/UH for watersheds. Multiple inflows routing model can integrate a number of upstream flows to compute the common downstream flow rate on the basis of several upstream flows. This book describes flood flow modeling for river systems having a number of gauged and ungauged watersheds. Flow contribution from the ungauged watersheds during the selected storm events are determined by applying GIUH and NRCS techniques. Watersheds are segmented into a number of sub-watersheds and geomorphologic parameters are estimated by applying GIS tools. UHs for the sub-watersheds are routed and superimposed to derive UH at the main watersheds outlet. Outflows from the gauged and ungauged watersheds are integrated and routed to estimate the inundation area per meter length of the channel at the common downstream section in a river system.