Tree-ring reconstructed precipitation and streamflow records in the western US show that present a strong ~20 year cycle from 1660 until approximately the last third of the 19th century, and a tendency towards lower frequency variations (multidecadal to centennial) starting at the beginning of the 20th century. This latter period of low frequency variations have been associated with the variations of the Pacific Decadal Oscillation.

We also identified a similar period dominated by low-frequency variations that occurred around 1550 to 1650. This period contains the most severe drought in terms of water allocation in the Upper Colorado River Basin (UCRB) identified through tree-ring reconstructions. The initial period of low frequency variation affected most of the UCRB and the southwestern U.S., while the most recent period of low variability seems to be affecting with more intensity the Upper Missouri and Yellowstone basins.

The tendency towards lower frequency streamflow variations in the last 100 years suggests an increase in the risk of severe and sustained droughts compared to the previous 200 years, especially if some of these climatic forcing mechanisms are a reflection of a natural mode of variation of the ocean-atmosphere system.