Water Supply Update

Eastern Idaho Water Rights Coalition Members:

Re. Water Supply Update

Last Thursday the Upper Snake River Advisory Committee heard reports from various weather, snow, and water agencies, plus Idaho Power.  While to many it may appear that the East Idaho drought is over, low streamflow, low reservoirs, and low soil moisture will still have a big part in the 2023 water supply picture.  Here are some highlights of the meeting, followed by some personal views.


  • Near and medium term outlooks continue to call for colder than normal temperatures and above normal precipitation.
  • The three month outlooks shows equal chances of wet / dry and warm / cold.
  • The long-term summer outlook is for warmer and dryer than normal.
  • Drought removal this spring is likely.
  • We are in a transition from the La Nina to an El Nino.


  • Low elevation snow, which is not measured as well or as often, may play a bigger role this year.
  • Soils across the watershed are still dry – which will soak up more of the snow pack.
  • Expect near normal streamflow.

Bureau of Reclamation (BOR)

  • Jackson Reservoir is the least likely to fill this year.
  • 98% streamflow is predicted at Heise.
  • The peak reservoir fill is expected to be 2.7 million acre-feet.  That is some 200,000 acre-feet more than last year, but still would only put the entire system at 67% full.
  • Ririe Reservoir and the Little Wood Reservoir will both have excess water to spill.  (Recharge water should be available on the Little Wood.)
  • River base flows are still low, but a little better than last year.
  • Irrigation is expected to begin in the Magic Valley around April 1.
  • Flow augmentation (for salmon flush) is expected to require 60,000 acre-feet of water this year, but could reach 150,000 acre-feet, depending on supply.

Idaho Power

  • Predict Heise flow to be 85% of normal.  (vs. 98% for the BOR) (The last couple of years Idaho Power was more accurate.)
  • Swan Falls and flows into Brownlee Reservoir are both at record low levels.

Personal Predictions  (I know, risky)

  • I believe the reservoir fill will be higher than predicted by the BOR for these reasons:
    • Snowfall has continued to be significant during March, and since the BOR predictions were made.
    • We are in a cold, wet pattern that shows no sign of ending.  Spring moisture and temperatures are very important.  Cool, wet weather continues to accrue snowpack, while warm, dry patterns can lead to irrigation demand starting in early April, even in East Idaho.  That will not happen this year.
    • The snowpack in the Willow Creek (Ririe Reservoir), Blackfoot River, and Portneuf River are 150% of normal.  There should be some high stream flows from those three Rivers to help fill American Falls. (Currently 68% full.)
    • Reservoirs on the Henry’s Fork are on track to fill in a timely manner.  With115% snowpack there, we should see higher flows from the Henry’s Fork to either help fill American Falls, or delay releases from Palisades Reservoir.

Bottom line, the water supply is certainly better than last year, but depending on weather the next three months, there could still be shortages.