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ABOVE AVERAGE JUNE RAINFALL LEADS TO BIGHORN FLOW INCREASES


Note: This blog post is an updated version of the 6/8 post and includes outlooks on the fishing by Bob Krumm, and the fishery by MFWP Fisheries Manager Shannon Blackburn.


Earlier this month, BHRA published a blog post on its website outlining the variables that affect river flows in June. The bottom line (on June 1st) was that we were not overly concerned about flows going up significantly but were keeping a watchful eye on Wyoming precipitation. At that time, we were coming off two months of below-average precipitation and sitting at below-average snowpack conditions for this time of year. Combined with below-average forecasts, 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 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. Fast forward to today, we are anticipating Bighorn River flows to increase to 11,000cfs – 12,000cfs by next week.


WHAT HAPPENED?

It rained. It rained a lot. The precipitation received in the Bighorn Basin during this time was significantly more than the typical June rainfall in Wyoming. In less than two weeks, since June 1st, the basin received over 120% of its average monthly precipitation (Figure 1). As a result, the June inflow forecast increased by approximately 200,000 acre-feet, with only about 100,000 acre-feet of storage capacity remaining in the reservoir. The excess water that cannot be stored must be released from Yellowtail.

Figure 1: Monthly precipitation accumulation for the Bighorn Basin. While April and May received below-average precipitation, June received over 120% of average precipitation for the month in less than 10 days.

WHAT CAN WE EXPECT FOR INCREASES AND DURATION?

In response, and at the time of writing, Bighorn River flows are rising from 8,500cfs to the anticipated range of 11,000cfs to 12,000cfs, where they are expected to remain until the first week of July. The National Weather Service predicts more precipitation over the next 10 days, but it is uncertain whether it will impact the basin, which has been the pattern this spring. Releases and durations are projections and are being adjusted daily based on updates on basin conditions and forecasts, so please note that this projection can change.


COULD THIS HAVE BEEN PREVENTED?

Water managers rely on forecasts, reservoir levels, and snowpack to make operational decisions. While reservoir levels and snowpack can be directly observed, spring rainfall forecasts, particularly in the Rocky Mountain region, have a significant margin of error. The National Weather Service and the Drought Monitor Index provide predictions, but the accuracy is limited. Coupled with changes in climate conditions over the past 30 years, accurately predicting spring precipitation is challenging. An audit conducted in 2019, requested by BHRA, revealed a mean average error of 400,000 acre-feet in the BOR's April to July forecast. Until long-term forecasts for late spring precipitation in the region improve, uncertainty regarding rainfall will persist. However, BHRA will be reviewing this year in detail with BOR and MFWP post runoff.


COULD THE RESERVOIR HAVE BEEN HELD LOWER?

We often face the question of why the reservoir wasn't kept at a lower level to mitigate uncertainty caused by rainfall. It's a challenging situation with no easy answer. If earlier dry forecasts had materialized and the BOR maintained high flows instead of reducing them, we would risk falling below the recommended 2,000cfs for the fishery during summer and jeopardize filling the reservoir necessary for winter flows for young-of-year wild trout. Additionally, it's important to consider that we have a reduced reservoir capacity of 8% due to sediment accumulation, further limiting BOR's flexibility in water allocation among stakeholders.


WILL THIS AFFECT THE FISHING?

To answer this question, we reached out to guide and Bighorn fishing legend Bob Krumm who has been fishing the Bighorn since 1985.

I have always contended that the fishing on the Bighorn River improves markedly when the water temperature reaches 45 degrees and really gets good when it reaches the low to mid-fifties. With these increases, we are seeing temperatures reach this range which in turn increases the metabolic rate and feeding activity of trout. My records show that if the water temperatures reach 55 in early July, Yellow Sallies hatched in large numbers from Bighorn to Mallards. They also worked their way up to 3-Mile. Incidentally, in 1999 I had 120 trout taken on Yellow Sally patterns in July. In 2000, when the water temperature never reached 55, I had zero trout taken on Yellow Sallies. Also, with warmer water the rainbow eggs hatch faster so the fry will be in the river sooner and have a chance to grow somewhat before winter sets in.


While some traditional hot spots inundated by the high flows will be “washed” out, others will be created. There is still wade fishing on the inside bends of the river, on the island edges, and side channels like Picture, Klein's, African Queen, and Juniper. I have had excellent dry fly fishing during high water for PMDs on the islands. The row around crowd will still find backwaters they can row back up in, or maybe they will have to double float. Break out the BB and 3/0 shot and start chucking and ducking!


WILL THIS AFFECT THE FISHERY?

To address this, below is a response from MFWP Fisheries Manager, Shannon Blackburn

As for the fisheries, Bighorn brown and rainbow trout respond differently to water events. The timing, magnitude, and duration of high or low water, coupled with winter flow releases and spawning periods influence abundance and size structure. Typically, brown trout (fall spawners) favor high water years, and rainbow trout (spring spawners) do better in average-to-low water years. The current forecasted flows will likely disrupt some spawners and scour some redds. However, Bighorn rainbow trout have an extended spawning season and some of this year’s juveniles have likely emerged from redds already. The late spawners may find suitable areas and contribute to the fishery as well. At these water levels, both juvenile brown and rainbow trout will be able to take advantage of novel habitats in side-channels that provide refuge from predators and mainstem flows.


High flows are not always detrimental to the fish population. From a habitat perspective, occasional flows exceeding 10,000cfs are beneficial to the river and fishery. High flows are needed to maintain channel complexity, prevent excess vegetation from growing in the side-channels, flush fine sediment from gravels, and move woody debris that creates pools, gravel bars, and riffles.


WHAT IS BHRA DOING?

BHRA is monitoring river conditions during this time and communicating with BOR and MFWP multiple times a day. As of this writing, 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. 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 duration, (past the anticipated first week of July) we will be monitoring the impacts and learning as much as we can. Lastly, we are 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 help spread out and slow down the river during high peaks. While we cannot predict much when it comes to spring rainfall, 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.


THE BOTTOM LINE

The bottom line is that it is impossible to have accurate long-term forecasts for rain. While we are not thrilled about the increase, we do not anticipate this being a “season-long” event. Starting today, we anticipate flows increasing in 500cfs increments, twice a day, until flows hit the 11,000cfs – 12,000cfs mark anticipated by BOR.* It is expected that flows will last at these higher levels until the first week of July. Fishing has remained incredible and fish health is great. As always, we will be communicating with water managers and fisheries managers, while also being out on the river learning as much as possible!


Thank you for helping us be responsive in learning more about the Bighorn River amidst changing conditions. We will continue to keep you updated.

*Spring conditions can change fast and flows can increase or decrease daily based on updated weather patterns and inflows. To stay up to date on current Bighorn flows, please check out the River Dashboard above. For daily updates on flow changes, please follow us on Facebook.



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