Tree-ring footprint of joint hydrologic drought in Sacramento and Upper Colorado river basins, western USA

Although flow in the two rivers is only very weakly correlated over the full 538-yr reconstruction period, more years of joint drought occur than would be expected by chance alone. Covariation in reconstructed flows is stronger in the late 1500s and mid-1700s than at any time since 1800. The late 1500s period of drought is not characterized as a decades-long unbroken drought, but as a series of drought impulses broken by wet years, with widespread moisture deficits in joint dry years. Periods of high inter-basin correlation in reconstructed flow are characterized by coherency at frequencies within the ENSO band. However, joint droughts in instrumental gage records do not display any consistent relationship with ENSO or the Pacific Decadal Oscillation (PDO), and so it is difficult to infer either as a causal mechanism for joint droughts in the past.