Water Supply Update

On Friday afternoon, just as I started writing my blog, I received a phone call from Idaho Department of Water Resources (IDWR) staff in Boise. The purpose of the call was to alert me that water-supply and administrative conditions were aligning to allow diversion of water upstream of American Falls Reservoir for aquifer recharge. That prompted me drop what I was starting to write and instead revisit the water-supply situation, which I last did on January 7. So, how are we doing five weeks later?

First, the reservoir system has continued to fill nicely, which is the primary reason why water will become available in the next day or so for aquifer recharge upstream of American Falls. As I mentioned a few weeks ago, diversion for aquifer recharge has been taking place downstream of American Falls since the end of irrigation season last November. However, no water can be diverted upstream of American Falls for recharge until a hydropower water right at Minidoka Dam and certain storage conditions are satisfied. The upper Snake reservoir system is now over 76% full, meaning that American Falls Reservoir storage rights are filled, and the storage rights in the system that are yet to fill are all upstream of American Falls, including those in Island Park Reservoir. The majority of the rights remaining to be filled are those held in Palisades Reservoir, and even though weather has generally been warm and dry recently, snowpack upstream of Palisades is slightly above average. In fact, the snow currently on the ground throughout the basin is sufficient to fill the entire reservoir system, both physically and on paper.

Before getting into the details of the water supply in the Henrys Fork watershed, let me mention that some of this so-called “upper valley” recharge will occur in the Henry’s Fork watershed, within the Fremont-Madison Irrigation District (FMID). Specific amounts, locations, and timing of diversion are still yet to be determined and depend on many factors such as water rights, canal capacities (even with warm, dry weather, most of the FMID canals are still full of snow), and recharge capabilities of other irrigation districts upstream of American Falls. Based on the information I received on Friday, total diversion for recharge from the Henry’s Fork and/or the lower Fall and Teton rivers is likely to be around 300 cfs. HFF has recommended that diversion for recharge not drop flow in the Henry’s Fork at St. Anthony below 1200 cfs. Flow at St. Anthony has been steady at around 1500 cfs, so 300 cfs diversion for recharge will maintain our recommended flow, providing an opportunity for everyone involved to demonstrate that aquifer recharge can occur without harm to the fishery. In fact, some of this recharged water will return to the Henry’s Fork downstream of St. Anthony later in the year, providing inflow of cool water during the middle of the summer and hence providing some benefit to the fishery.  

If water-supply and water-rights conditions allow 300 cfs diversion from now until the beginning of irrigation season, FMID could recharge over 25,000 acre-feet of water to the Eastern Snake Plain Aquifer, adding to the amount that has already been recharged downstream of American Falls so far this winter. As I’ve mentioned before, aquifer recharge alone may not be sufficient to address all of Idaho’s water-supply challenges, but it is necessary to stabilize the Eastern Snake Plain Aquifer and provides a storage mechanism that has much lower environmental and capital costs than building new storage reservoirs. Everyone involved—from HFF to the Idaho Water Resource Board—will be monitoring the situation closely to document when and where recharge occurs and at what effect on streamflow.

Ok, now on to the water supply picture. Although the outlook over the whole upper Snake River basin is reasonably good, it has gotten quite a bit worse in the upper Henry’s Fork watershed since I last reported on the situation in early January. Henrys Lake inflow has been tracking the long-term median pretty well, and if that continues between now and April 1, inflow to Henrys Lake will average about 40 cfs. The lake is essentially full right now, at 21 cfs outflow, so Henrys Lake outflow will need to be increased relatively soon to match inflow and avoid over-filling the lake.

Unfortunately, inflow to Island Park is not as good. It has been about half-way between the 25th percentile and the median, definitely on the dry side. Assuming this trend continues from now until April 1, average streamflow between Henrys Lake and Island Park will be 395 cfs. Adding the 40 cfs from Henrys Lake, total inflow to Island Park Reservoir will be 435 cfs. The operational objective agreed on by the Island Park Drought Management Planning Committee last fall was to attain 127,000 acre-feet (94% of capacity) on April 1. From where we are right now, that requires storage of 289 acre-feet per day. With 435 cfs inflow, the fill objective will be met with an outflow of 288 cfs at Island Park Dam. Outflow is currently around 280 cfs, so we are right where we need to be. The remaining 8,000 acre-feet necessary to bring the reservoir to 100% of capacity cannot be stored until after the reservoir is ice-free, which is why the objective is only 94% of capacity by April 1 instead of 100%. The last 8,000 acre-feet can easily be stored as the snow melts, even with below-average snowpack.

Speaking of which, snowpack in the upper Henrys Fork is indeed below average. Snow-water-equivalent at Snotel sites in and near Island Park is around 85% of median (remember that this is only 82% of the mean), and the Crab Creek site, which I have identified as a statistically significant predictor of spring runoff to the west end of Island Park Reservoir, is currently only at 61% of median. The two-week weather outlook is for warm and dry conditions, so my forecast for below-average inflow to Island Park Reservoir this spring is becoming more likely by the day. I’ll wait a few more weeks to put some numbers on this.   

Regardless of the weather, we do not want to risk what happened last spring, when Island Park Reservoir did not fill. Remember that this resulted in very low outflow from Island Park Dam early in the fishing season, a situation we do not want to repeat. Because of the difference between physical water in the reservoir and storage water rights (what we call “paper water”), failure to fill Island Park Reservoir did not have any water-rights consequences for FMID last year. This year, however, failure to fill Island Park Reservoir would have negative consequences for FMID. So it is in everyone’s interest to stay on our trajectory of physically filling Island Park Reservoir as early as possible this spring. This will become even more critical if weather stays mild and dry and irrigation demand in the lower valley is high in April.

In the meantime, let’s hope for a lot of rain and snow in March.