The flood hydrology of the Binational Cottonwood Creek - Arroyo Alamar, located in California and Baja California, has been studied. The majority of the contributing watershed (86.2 percent) is located in San Diego County, and the remaning portion (13.8 percent) lies within the municipalities of Tijuana and Tecate, in Baja Caifornia.          The objective is the calculation of flood flows for return periods from 2 to 1000 years. The analysis has been made using a conceptual hydrologic modeling of rainfall-runoff, of general aplicability. The principal characteristics of the model are: (1) generalized topology, capable of handling a drainage network of any order; (2) rainfall specified for each subbasin, either upland or reach; (3) hydrologic abstraction with the SCS runoff curve number; (4) conversion of rainfall to runoff by means of the convolution of the effective hyetograph with the SCS unit hydrograph; (5) reservoir routing with the storage indication method; (6) channel routing with the Muskingum-Cunge method; and (7) calculation of channel transmission losses for flood propagation in ephemeral streams.          The model was run to determine flood discharges for return periods from 2 to 1000 years, specifying the maximum of the mean monthly levels for the period of record as initial condition in Morena and Barrett dams, which control the flow of Cottonwood Creek, in San Diego County. The flood discharges calculated for return periods of 2, 5, 10, 25, 50, 100, 200, 500, and 1000 years are: 280, 530, 680, 930, 1140, 1310, 1420, 1600, and 1720 m3 s-1, respectively. For the delimiting of federal zones, the design discharge should be greater than 680 m3 s-1, corresponding to a 10-yr return period. For the design of the channel improvements, assuming the applicability of agricultural zones with urban settlements, the design discharge should be greater than 1140 m3 s-1, corresponding to a 50-yr return period, and less than or equal to 1420 m3 s-1, corresponding to a 200-yr return period.