The Upper Snake River System will End the Season in Better Condition than the Previous Nine Drier Years Since 1966

Last spring, we posted about lower than average snowpack in mountains of eastern Idaho and northwestern Wyoming following a drought year in 2021, potential reduced water available for irrigation in central and eastern Idaho and parts of Oregon, and what the severity of the shortage might be relative to history. The graphic below is an update of one we published at that time, which puts into perspective the current drought in eastern Idaho and western Oregon as it relates to surface water irrigation availability relative to previous years.

To review, the blue line is the daily total of water stored in the dam system of the Upper Snake River in eastern Idaho and the northwest corner of Wyoming. The oscillation of that line shows the annual seasonal change in that inventory as the total stored water hits its lowest levels in early October (box 1), builds into the following spring and early summer, and then decreases through the remainder of the summer as water is released from those dams for irrigation and other uses.

In the light-yellow box at the bottom of the graphic, nine years saw lower minimum levels than that in October of 2021 (box 2). The question then becomes how much will water inventories recover before usage in the following summer. Some years such as 1992 recover to high enough levels that there is no apparent shortage for irrigation the following summer. Others, such as 2002 – 2004 do not fully recover.

Alternatively, years of abundant water availability (box 3) peak in their stored volume later in the year (typically mid-June to mid-July) than do years of low water availability (box 4, April to early June). This makes sense in that we might expect higher levels of snowpack in the surrounding mountains to take longer to melt and flow into the dams than years of lessor snowpack.

So how did this summer play out? As expected, it was not the worst on record, with peak inventory not occurring until 6/21/22 (box 5). But how does this look relative to other dry years?

The 2nd chart below compares the daily water inventory levels of 2022 relative to the other dry years recorded since the mid 1960’s, with the red line representing 2022. From left to right, we begin with each year at about its peak inventory date (box 1) and then follow it to its lowest inventory date on the far right (box 2). Most of these dry years immediately begin drops in inventory from that point or early May, 2022 by contrast, was relatively steady in April and early May and then built additional inventory starting at about May 17th, presumably on late spring rains in eastern Idaho which culminated in the Yellowstone Park flooding on June 12th, and then peaked on June 21st. The late rains moved the 2022 inventory from having the second lowest peak since the mid 1960’s to maintaining a higher daily inventory than the previous nine driest (box 3). As such, we would expect the 2022 season to finish with at least as much and as late as those previous dry years. To state the obvious, the 2023 season will be riding on this winter’s snowpack.