Annual natural discharge (Q) of the River Murray and its most extensive tributary, the Darling River system, is often inversely related to sea surface temperature (SST) anomalies in the eastern tropical Pacific Ocean. These SST variations are components of a planetary-scale phenomenon referred to as El Nino--Southern Oscillations (ENSO). Darling and Murray river historical values of Q indicate that annual surface runoff from regions dominated by subtropical summer monsoon precipitation and annual surface runoff primarily responding to temperate winder storms are both strongly influenced by ENSO cycles. Forecasting, approximately 1 year in advance, of ENSO-related SST from geophysical model calculations thus provides a mechanism for estimating probabilities of annual river discharge amount. Contingency tables relating annual Q to SST, based on combining observed data for 95 years and forecast SST over a period of 15 years, provide probabilities of expected annual Q as a function of forecast SST. The SST of the eastern tropical Pacific was successfully forecast to be appreciably warmer than long-term mean conditions for much of the year beginning in mid 1991. Precipitation data through 1991 indicated that annual natural Q for the Darling River was probably substantially below the mean. However, winter precipitation in higher-runoff portions of the Murray Basin was above average during this El Nino episode, contrary to the trend for most such events over the past century. ¿ American Geophysical Union 1993