A stochastic streamflow program was developed to simulate inflow to a large river from a network of gauged tributaries. The program uses historical streamflow data from major tributary gauges near their confluence with the main stem and combines them stochastically to represent spatial and temporal patterns in flood events. It incorporates seasonality, event basis, and correlation in flood occurrence and flood peak magnitude between basins. The program produces synchronous tributary inflow hydrographs, which when combined and routed, reproduce observed main stem hydrograph characteristics, including peak, volume, shape, duration, and timing. Verification of the program is demonstrated using daily streamflow data from primary tributary and main stem gauges in the Sacramento River basin, California. The program is applied to simulating flow at ungauged main stem locations, assessing risk in fluvial systems, and detecting bed level change.