With warm weather and premature mountain snowmelt, what's going on in Idaho's reservoirs?

The study would cost $5.5 million and look at different ways of increasing the Treasure Valley water supply, including raising Arrowrock Dam 10 feet, Anderson Ranch Dam 6 feet or the Lucky Peak Dam pool 4 feet. (File Photo)

The winter of 2016-2017 was anything but normal. One could technically say the same thing about this winter season, although the comparison to 'normal' is coming from polar opposite directions when talking about 2016-2017 vs. 2017-2018.

Thinking back to last year

Mid-February was an interesting time in 2017. Idaho's mountains had seen heaps of snow, and several SNOTEL sites (spots where snow depth is measured) were on their way to setting records for snow depth by early March. In fact, some SNOTEL sites reported seeing nearly two winters's worth of snow last season. Eventually, though, we know this caused a "bottleneck effect" of sorts in the state's reservoir systems, leading to flooded rivers and a balancing act that prompted many people to become familiar with terms like 'cfs,' or, cubic feet per second, and the state's 'Tea Cup' graphs. (The Boise Basin graph, for example, gave daily updates on on how much water was sitting in Lucky Peak, Arrowrock and Anderson Ranch, along with how much water was being shoved out into the Boise River system as a result of storage levels).

Mid-February, though, was particularly interesting in 2017, because that was the point in time where water managers began releasing more water from the Boise Basin's reservoirs and into the Boise River in attempt to preempt the massive surplus of water that would eventually melt from the mountains and into those reservoirs.

Notice, in the graph above, that very little water is currently being discharged into the Boise River through the Diversion Dam this year (blue line). That's similar to what happened through the winter months last year (green line), although on an average year, we tend to see flows increase slowly into and through the Boise River system beginning in January, and continuing gradually through March or April (red line). Typically, flows then increase more substantially as spring wears on (between April and June, especially).

Last year was different though. When mid-February hit, water managers made the decision to start releasing more water into the reservoir system, quickly. Hence, the huge spike you see in the green line (which represents last year's data). River levels rose above flood stage (approximately 7,000 cfs) along the Boise River before mid-March, and stayed there through mid-June. You remember the rest.

How will this year be different?

Looking at the graph above, I'm not anticipating that blue line to follow the path of the green line at all. In fact, there's a possibility water managers could decide to keep it below the red line (representing the overall average trends). Why? Because there's no need to "get ahead."

Snowpack numbers for February 8 show that most of the basins in southern Idaho are seeing between 50 and 85 percent of the normal "snow to water equivalent," (or, more simply put, amount of snow compared to normal). That means we don't even have what's considered an 'average' snowpack in the mountains right now. Thus, we're not preparing for an unusual amount of water to melt and filter through the reservoir system and eventually down the river. Instead, this year, water managers could actually decide to "hold on" to some of the water in those reservoirs as long as possible before releasing it through the river system.

There's no flood threat waiting on the other side. In fact, this year, the "balancing act" will have nothing to do with preventing flooding, but instead, helping to ensure that the water we do have gets delivered down the river system gradually through the summer to help farmers and recreationalists alike enjoy the benefits all summer long.

What's going on in the reservoirs now?

If you're curious about what is going on in a particular reservoir in the Boise Basin, I've attached photos in the photo gallery link located at the top of this article that breaks down the picture for Anderson Ranch, Arrowrock and Lucky Peak. However, the graph below gives a general consensus of a 'graphical look' at this year's water year in the reservoirs.

What does this mean? Looking at the graph, it's evident that there is much more water in the three reservoirs collectively this year in comparsion to last year or compared to average. The blue line (this year's data) has been climbing steadily through the months of January and February. While that's not uncommon, it does look as if the blue line is going up at a faster rate than the red one. That's because we're seeing more snowmelt at this point in the season than we should during a normal year.

Warm temperatures in southwest Idaho have prompted some snowmelt in the high country, leading to faster flows on the Boise River near Twin Springs for example. (This was most evident with water flowing into Arrowrock Reservoir earlier this week in response to warm temperatures over the weekend, as noted below). This trend is gradually pushing additional water (even if it's not much) into the reservoirs.

In theory, it's not necessarily problematic to have more water sitting in the reservoirs right now, but what is (in theory) potentially problematic is that this process (of snowmelt and reservoirs filling up as a result) is happening ahead of schedule. We need a change in the forecast to make sure this doesn't continue for too long. Here's where our reservoirs are at right now:

Is there concern about a water shortage this summer?

Because there is still so much time left in the 'winter season', the short answer here is that it's impossible to predict what this year's water year will look like and exactly how much water we will or wont have to work with as the season changes from winter to spring and then to summer. However, the good news is that this year, we started the water year off with a serious buffer from last year.

If you notice this year's data below in the Boise Basin graph (blue line), you'll notice now that its starting point is significantly higher than last year or compared to the average. That's thanks to the extra water in our reservoir system that was 'left over' from last year. (See...there was a silver lining to everything that happened last winter and spring!)

The process of releasing water through the reservoir system and into the Boise River could have to begin sooner than usual this year, because if snowmelt continues to happen prematurely in the mountains, the reservoirs would then fill toward capacity sooner. That being said though, this won't necessarily spell a death sentence for our water situation this summer. Again, that's thanks to the 'buffer' we got last year from a record-breaking winter.

As for how and when that water in our reservoirs right now starts getting pushed through the river system...well, that's up to water managers. Stay tuned, as we'll be discussing this more next week.

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