Author: Anne Marie Emery
On June 2nd, we published a blog post outlining the variables that affect river flows in June. The bottom line was that we were not concerned about flows going up significantly but were keeping a watchful eye precipitation occurring in Wyoming. At that time - just 6 days ago – we were coming off two months of below average precipitation (Figure 1) and sitting at below average snowpack conditions for this time of year. Combined, these variables inferred that dry conditions would likely challenge the Bureau of Reclamation’s ability to keep river flows above the MFWP (Montana Fish, Wildlife & Parks) summer fisheries flow target of 2,500cfs. In result, discharge from upstream dams and Yellowtail were either maintained or decreased to start preserving water for storage – storage that would enable flows being kept above the 2,500cfs threshold throughout the summer while simultaneously storing water for sustainable winter flows that are vital to young-of-year wild trout.
Over the weekend it rained, both in Montana and Wyoming. It rained a lot. While Montana received greater amounts of rain, the moisture needed to generate the recent rainstorms was much more than Wyoming typically sees in June. On Sunday, parts of the basin in Wyoming incurred 0.69 inches of rain in 15 minutes! The result was up to a 5-foot river rise in some tributaries in less than 3 hours. In total and since June 1st, the basin has received over 60% of its average precipitation in less than 7 days (Figure 1).
In result, and in a short period of time, the June inflow forecast increased by about 200,000 AF, with approximately 100,000 AF left of storage space in the reservoir. This additional volume is based on past and current and forecasted conditions. The water that cannot be stored has to be released from Yellowtail.
Figure 1: Monthly precipitation accumulation for the Bighorn Basin. While April and May received below average precipitation, June has received 60% of average precipitation for the month in 6 days.
COULD THIS HAVE BEEN PREVENTED?
Water managers base operating plans on forecasts, reservoir levels and snowpack. While reservoir levels and snowpack are observable, rainfall forecasts (especially in the Spring) are limited to what the National Weather Service and the Drought Monitor Index can accurately predict. Currently, these prediction models have a large margin of error for spring precipitation, error that when coupled with changes in the 30-year average climate conditions, are hard (in all places) to get right. Until long-term forecasts for late spring precipitation can be improved (worldwide), there will always be uncertainty with rainfall.
COULDN'T THE RESERVOIR HAVE BEEN HELD LOWER?
Sure, but if the forecast had remained dry and river flows were not decreased, we could be seeing summer flows drop below 1500cfs for the Bighorn and risk the reservoir not filling which would impact winter flow releases that are vital for young-of-year trout survival. BOR has to be responsive to current conditions and be guided by forecasts, which as mentioned, are hard to get right with rainfall predications.
Release schedules are being adjusted daily based on updates to basin conditions and forecasts. Currently, river flows are increasing daily until river releases reach 7,000cfs on Saturday. The National Weather Service for the upcoming 10 days is forecasted to bring in close to what we experienced a last week in Wyoming. If this pattern of precipitation events continue, BOR will have to further increase releases to offset the additional volume of water. At this time, and without the June Operations Plan available, we are unable to make predications on how high, or for how long we will go. We do hope for relief in this wet pattern, soon.
WHAT IS BHRA DOING?
BHRA is monitoring river conditions during this time and communicating with BOR and MFWP multiple times a day. As of the writing of this post we have interns actively collecting water quality data on both the reservoir and river and are using the BHRA drone to monitor spawning complexes and conditions. As mentioned in the previous post, spring pulses, or freshets of reasonable magnitude and duration, can be beneficial for the Bighorn River by cleaning out vegetation, giving a boost to temperatures (as water is released from higher in the reservoir) and increasing macroinvertebrate diversity. However, if the peak increases in magnitude and duration, we will be monitoring the impacts and learning as much as we can. We are also busy finalizing contracts for our large multi side channel restoration work that will occur in the fall. When complete, this work will help the river and its trout by opening vital habitats that will stay connected to flow at a minimum discharge of 2,000cfs, and spreading out and slowing down the river during high peaks. While we cannot predict the duration of this 7,000cfs release, or conclude if more flow increases will occur, we are out there learning as much as we can through it all and working on projects that improve the resiliency of the fishery through changing climate patterns.
Thank you for helping us be responsive in learning more about the Bighorn River amidst changing conditions. We will continue to keep you updated.
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