Gem County, ID

Snowpack 

David Hoekema
Hydrologist Idaho Dept. of Water Resources

Snowpack is off to a strong start in southern Idaho, with the snowpack south of Salmon River being equal to or greater than 120% of median.  However, we are still early in the snowpack accumulation period; snowpack will likely peak around April 1^st.  A review of snowpack and water supply conditions in the Boise River and the Upper Snake will help to develop an understanding of current water supply conditions in Idaho, which have been completed below:

Boise River Water Supply 
Last year we experienced a very unusual climatic event allowing the Boise basin to emerge from drought with less than average runoff.  This was caused by low spring temperatures and above average precipitation in April and May, which delayed runoff and agricultural demand. 

Now we have ideal conditions with above average storage (Figure 1) high in the system. Specifically, difficult-to-fill Anderson Ranch Dam (Figure 2) has a high likelihood of filling.  Full storage allocations on the Boise River seem likely, even if below normal runoff occurs.  Currently, snowpack is around 134% of normal and 57% of median annual peak.   Thus, it seems likely that the Boise basin will have an adequate water supply for next season. 
One significant factor that could derail this prediction would be extremely dry and/or warm conditions in the spring.  We did not receive ideal precipitation conditions in October and runoff may be less efficient than normal.

Figure 1     Reservoir Conditions on the Boise and Payette River are ideal.  Most of the storage in the system is held high up in Anderson Ranch Dam with storage space available in the lower system should flood control operations become necessary in the spring. 

Figure 2     Current storage in the Boise River system (blue line) is above average (red line) for this time of year, and about 175,000 ac-ft above last year (green line).

Figure 3     Snowpack is in the Boise Basin as measured by the NRCS Snotel Network.  Median annual peak snowpack is shown by the green “x” with current water-year snowpack shown by the black line.

Snake River Water Supply Conditions 
Conditions on the Snake River mirror conditions in most of southern Idaho (aside from the Boise/Payette system), and high drought risk continues in the Upper Snake River basin. The reason for drought risk is that a) Snake River storage is about 1,000,000 ac-ft less than normal (Figure 4), b) there is low carryover storage in reservoirs high in the system (Jackson Lake and Palisades) which are more difficult to fill and c) critical storage is even less than last year. 

To recover from drought conditions, we most likely need current high snowpack levels (124% of median) to be maintained because currently we are only 51% of median annual peak snowpack (see Figure 5).  For drought recovery, we will need a continuation of current storm activity until the end of the accumulation period.  At this point, it appears that drought will continue across most of southern Idaho next year, with the exception occurring in the Big Lost River basin due to high carryover from last year.  Note that snowpack accumulation is not far from where it was last year, when the storm systems turned off on January 8^th.  Without the cool spring in eastern Idaho, we could have had a record setting drought.


Figure 4     Current Storage in the Snake River System (blue line) is about one million acre-feet storage of normal conditions (red line) and even less than in 2022 (green line). 

Figure 5   Snowpack in the Snake River Basin above Heise with the green “x” representing median annual peak snowpack, the black line representing Snowpack in the current water-year and the orange line representing snowpack in WT-YR 2022. 

Current Forecasts 
A trough off the Pacific coast means an active weather pattern is likely to continue for the next 10-days. 

La Nina Trends 
La Nina conditions in the equatorial Pacific however point to drier than normal conditions in southern Idaho, based on historic trends.  Since the 1950s consecutive La Ninas have resulted in less snowpack.  However, there are only a few times we have had three La Nina winters in a row (once in the 1950s, 1970s, 2000s, and now.  One of the consecutive La Ninas in the 1950s was actually wetter than the previous year.  Let’s hope that is what we get this year.  We are hoping a strong ridge does not develop off the Pacific Coast in late January and February blocking winter storms, as occurred last year.