The dominant regions of interannual streamflow variability in the United States are defined, and their seasonality and persistence characteristics identified, using an orthogonally rotated principal components analysis (RPCA) of a climatically sensitive network of 559 stream gages for the period 1941--1988. This classification of streamflow regimes is comprehensive and unique in that separate analyses of the streamflow record, for each month of the year, are carried out to detail the month-to-month changes in the dominant streamflow patterns. Streamflow variations, or anomalies, in the Upper Mississippi, South Atlantic/Gulf, Far West, Ohio Valley, Northeast, and Eastern/Mid-Atlantic regions, as well as a pattern of opposing streamflow anomalies in the West, are observed in all seasons of the year. Anomalies in the Southern Plains and New England regions are observed in autumn, winter, and spring; those in the Rocky Mountains and Middle Mississippi regions occur in late spring and summer.