A River Runs Through It

The Colorado River, to be precise.

Rights of American states and Mexico to the waters of the Colorado has been a contentious issue for decades.  With the U.S., the upstream states (Colorado, Wyoming, and Utah) have different interests than the downstream states (California, Arizona, and Nevada) and the downstream states are quarreling among themselves.  It's become particularly contentious because of the prolonged drought over the past two decades.

Currently, water rights for each state are determined by the 1922 Colorado River Compact.  Unfortunately, the water levels used as the basis for the allocation occurred during an unusual period of heavy rainfall from 1905 through 1917.  How unusual is described in a recent research paper by NASA scientists on the "Causes and Dynamics of the Early 20th Century North American Pluvial".

From the opening of the paper:

The 'Early 20th Century North American Pluvial' refers to a period of enhanced moisture availability across Western North America that occurred in the first two decades of the century and often delimited by 1905 to 1917. It has a peculiar and interesting history as it directly preceded the 1922 Colorado River Compact that began the legal division of Colorado River flow between states. The division was based on several years of measured flow that, because they occurred during the Pluvial, were much higher than anything that has occurred since. 

They conclude that the Pluvial was not part of the "normal" boom and drought cycle in the Southwest, rather it was an extraordinary event as they report:

To conclude, the early 20th Century pluvial was a unique event in the last 500 years of North America hydroclimate history and it is a great irony that it just so happened to precede the 1922 Colorado River Compact. Unlike the great North America droughts, which can almost entirely be understood in terms of reductions of precipitation, the Pluvial arose from a combination of wet conditions in the Southwest and cool temperatures across the continent. Also unlike the droughts it is not well simulated by a model forced by historical SSTs indicating less oceanic control over it than is typical for droughts. An important topic of research needs to be to explain the cold temperatures during 1905-17. Nonetheless western North America has steadily warmed since the Pluvial which, together with the fact that models predict much of the Southwest and Plains to dry as a consequence of how rising greenhouse gases impact the hydrological cycle, makes it exceedingly unlikely that similar moist conditions will ever return.