Each year, Bitterroot River Water Commissioner and damtender Al Pernichele and FWP fisheries biologist Chris Clancy get together to hatch a plan for how the “contracted water” in Painted Rocks Reservoir will be released. The reservoir, at full capacity, actually holds about 32,362 acre feet of water, but only 25,000 acre feet is under contract. Members of the Painted Rocks Water Users Association hold a collection of 41 contracts for a total of 10,000 acre feet of irrigation water. The Montana Department of Fish, Wildlife and Parks owns 15,000 acre feet of water for fisheries. The contract waters are released strategically throughout the season to try and keep the irrigation ditches flowing and at the same time keep the river flowing full enough to support a fishery. Every year it is a balancing act.
On September 7 of this year, there was about 6,900 acre feet of “contract water” left in the reservoir and FWP’s water was due to run out on September 12. The irrigators, on the other hand, had about 4,500 acre feet remaining. As a result, according to Pernichele, the outflows will be increased by an additional 50 cubic feet per second to allow for all the irrigation to be used by October 1. At that time, the outflows must match the inflows to the reservoir from Blue Joint, West Fork, Overwhich and Slate, somewhere between 90 and 100 cfs.
When river levels drop too low, the mainstem at Bell Crossing can even run dry, so that site has been the focal point for establishing a compromise between irrigators and FWP in the use of the reservoir’s water. According to FWP, ideally the river at the crossing should be 400 cfs or greater to ensure a healthy fishery. It can rarely be maintained at that level, however. It can easily get down into the low 200’s on a dry year. But this year, right now, it’s running at 350 cfs.
The problem with low flows is that shallow water warms up faster than deep water and trout don’t do well in warm water. But using the cold deep water in Painted Rocks Reservoir to cool off the river is a complicated business and may not be that effective, although it can have a moderating effect on the West Fork, according to Clancy.
He explains in a recent email the difficulties involved based on some scrupulous temperature measurements at four sites, on the West Fork below Painted Rocks, West Fork at Conner, East Fork at Conner and Bitterroot River near Wally Crawford FAS. He notes that during high flows in June and early July, water temperatures are not very different at the four sites. Usually, water temperatures peak sometime in late July to early August. It has peaked as early as the first week of July but that is rare. As day length decreases, water temperatures decline through August and September.
Clancy notes that water coming over the top of the spillway at the dam is warm. He has measured it at 71 degrees F on a hot day. In July, when water is released from below the spillway, it is at about 45 degrees F.
What is interesting, he said, is that you do see a decrease in the water temperature at Conner, but it has warmed up some and at Wally Crawford the decrease is less still. In fact, the Bitterroot River at Wally Crawford is not a lot cooler than the East Fork. So, the cold release water helps the West Fork, but not as much on the mainstem, at least down at Wally Crawford.
Another interesting thing is that the cold water only lasts a few weeks and then it starts to warm up as the reservoir drops and the coldest deep water is gone. This is because lakes and reservoirs stratify with the coldest water on the bottom, a narrow thermocline in the middle where water temperature increases quickly, and the warmest water on top. So, releasing it early would not be a good idea as it could use up the “reservoir” of cold water before it’s really needed in late July and early August. If water temperature was the only reason for using Painted Rocks water, they would probably release it a week or so later than they do. But, remember that the releases are aimed at streamflows at Bell Crossing, not so much the upper river. On years like this, when there is a lot of water and Bell Crossing is in good shape, it does allow Painted Rocks water to be used for other purposes such as water temperature moderation in the West Fork and a little in the upper river as well as floating boats in the upper river.
“Most years we do not have that luxury,” said Clancy.
Pernichele said, “It’s been a good year for water. In fact, this is probably the best year I have ever seen since taking the job.”