Water balances for the Colorado River mainstem complex, the Cienega de Santa Clara, and El Indio wetlands were calculated for the Colorado River delta in Mexico for the period 1992-1998. Discharge for the mainstem complex was disaggregated into flood and non-flood years, reflecting the marked variability of mainstem discharge at the Southerly International Boundary (SIB) delimiting the United States and Mexico. In non-flood years, agricultural and municipal returns to the mainstem below SIB contributed 180% of mainstem discharge at SIB, but may not be sufficient to generate the floodstage discharge required by native riparian vegetation.