Tricky year for reservoir managementJun 21, 2017 By Katie Roenigk, Staff Writer
Record-setting snowfall and subsequent melt-off has sent huge amounts of water into the water system.
Area water managers have been working together this spring to deal with record inflows entering local reservoirs.
More than 20,000 cubic feet per second of water flowed into Boysen Reservoir on June 10 - a new daily record according to Mahonri Williams, chief of the resources management division at the Wyoming area office of the Bureau of Reclamation.
Inflows have fallen since then, but they're still high. Williams said 18,900 cfs came in to Boysen on June 19, down to 18,500 cfs June 20 and close to 15,000 cfs June 21.
In fact, in just the month of June, Williams said Boysen has gotten more water than it usually receives during the entire runoff season, which includes April, May, June and July.
The 30-year average for April-through-July runoff is 540,000 acre-feet, Williams said. As of June 22, Boysen had taken in more than 608,000 af in June alone.
"And the month isn't over," he added.
The previous months' inflows were high, too, with water levels exceeding their average amounts in both April and May.
In total, Williams said the reservoir already has received 1.1 million af this spring - more than twice the average for the entire season - and there's still more than a month of runoff in store.
The Bureau's most recent forecast anticipates a total of 1.25 million acre feet of water will come in to Boysen this season.
To prepare for the inflows, the Bureau drew water levels at Boysen down to 60 percent full in May.
Last week, Williams said Boysen is more than 100 percent full.
"We're actually in the flood pool, as of midnight last night," he said June 22. "And we're still rising."
Boysen is only releasing 9,000 cfs of water a day, Williams said - less than the amount coming in, but more than usually goes out.
The outflow rate is determined in coordination with the Army Corps of Engineers, which considers the flood risks for local communities downstream of Boysen.
"We can go higher in our releases, but the desire is to not go any higher than necessary," Williams said.
He noted that it's not unheard of to utilize the flood pool, which had taken 1.3 feet of water June 22. In 2015, he recalled, the pool took more than 3 feet of water.
"We've been in this situation before," he said. "That happens every few years. It just depends on ... how high the inflows are and what the downstream conditions are."
The flood pool isn't separate from Boysen, he clarified.
"It's a certain elevation range in the reservoir," Williams said. "So as the reservoir comes up to a certain point it's in the flood pool, (or) a storage space specifically for flood control."
At these levels, he said, Tough Creek Campground at Boysen State Park sometimes is impacted by flood waters. The area was closed last week due to snowmelt runoff.
Upstream of Boysen, Williams said the Midvale Irrigation District has been doing a good job of managing outflows from Bull Lake.
He noted that the mountain reservoir doesn't have a flood pool, so it requires a different approach from Boysen.
"Once (Bull Lake) is full, whatever comes in, they have to let out," he explained. "(So) when the flows are really high, they pass some of that water through. ... They have an incremental difference between inflows and outflows so they're steadily filling the reservoir."
Midvale manager Jon Howell has resisted pressure from residents who have asked him why he didn't store more water earlier this year to mitigate flooding.
"If we'd done that we'd be in trouble now," Howell said. "We'd have to pass every drop of water (through)."
On June 22 alone, that would've meant sending 3,300 cfs downstream.
"Nobody wants that right now," Howell said.
Williams pointed out that Bull Lake Creek, which feeds Bull Lake, originates high in the Wind River Mountains, so it often continues to deliver snowmelt runoff through July. That's another reason it's prudent not to let the reservoir fill early in the season.
"(If) they fill it too soon ... and another inflow peak would come in late June or early July, they'd have to pass that right down the river," Williams said. "They're ... keeping some space so they can store that (water) and reduce the amount of flow going down."
In addition, Howell noted, there is an element of uncertainty that managers must take into account. For example, he said, on June 15, inflows into Bull Lake were one-third what they were this week.
In other words, the inflow level tripled over the course of one week.
"We have to look at that basically minute-to-minute, hour-to-hour, and make adjustments as necessary to protect the dam," Howell said. "Because, and I'll just state the obvious here ... if we think we've got flood waters now, it'd be like ridiculous if something happened to the dam. That's the No. 1 priority. We have to protect that."
The Bureau of Reclamation gives the Midvale Irrigation District target water levels to reach at certain times of the year for Bull Lake. The next target is 80 percent full by the end of June.
The reservoir was almost 75 percent full June 22.
Some residents have also wondered whether Pilot Butte Reservoir - or Morton Lake - could have been emptied earlier in the year and then used to store floodwaters and reduce impacts downstream.
Both Howell and Williams agreed that the system simply isn't designed to work that way.
Morton is an off-channel reservoir served by an irrigation canal that diverts water from the Wind River, Williams explained. Water is then delivered from the reservoir through a canal system to local irrigators as needed throughout the summer.
"If there isn't a demand for irrigation water from Pilot, the water just sits there," Williams said. "The normal operation is to have that reservoir pretty full and carry it through the winter so you have that water supply for the following spring."