The understanding of seasonal variations in streamflow is important for water resource management. The dynamics of streamflow are often dominated by annual and intra-annual variations, and the global warming debate has also generated an interest in potential changes in the seasonal cycle of hydroclimatic variables. Thus there are mechanistic as well as policy motivations for an empirical analysis of the historical seasonal variations in streamflow. The seasonality of the upper Mississippi River streamflow is investigated in this paper using a 123-year record of daily flow. This long streamflow series provides an interesting look at the high- and low-flow seasons within the year, their interannual variation, and within-season attributes. Evidence for changes in the timing and amplitude of these seasons and the annual flow extremes is presented. Connections to similar trends in regional climate variables are noted. The upper Mississippi River streamflow exhibits bimodal probability distributions for monthly averages and for specific seasons. Transitions across the high- and low-flow regimes corresponding to these modes exhibit memory across seasons and over years. Thus an empirical basis for seasonal or longer prediction is provided. Needs for developing a mechanistic explanation of the empirical observations offered are also indicated. ¿ 1999 American Geophysical Union