ACTION ALERT: Over the next two days, or until Saturday morning, Bighorn River temperatures are expected to be above average for this time of year. To help reduce additional stress on the fishery during this period, BHRA and MFWP ask all Bighorn anglers, guides and outfitters to practice quick catch and release protocol and avoid the overfishing of highly targeted areas on the river.
WHY ARE BIGHORN RIVER TEMPERATURES HIGHER THAN AVERAGE?
While it is not common for the cold-water section of the Bighorn River to experience elevated water temperatures, it can happen in high precipitation years, like this one, when the reservoir is filled rapidly with rainfall, and impounded waters do not have time to "sink, settle and cool." Currently, impounded reservoir waters being drawn through the turbine intake of the Yellowtail Dam are sitting 10-18 degrees warmer than in 2021 and 2022, equating to higher-than-normal river temperatures (Figure 1).
Figure 1: The BHRA RI program has been monitoring stratification of Bighorn Reservoir since 2021, with interest in how stratification of impounded waters varies between different water years, and how that stratification can affect downstream river conditions based on the specific dam outlet of which that water is pulled (i.e. Spillway, Turbines, or River Outlet). August sampling of the reservoir showed reservoir water being pulled from the turbine intake elevation as being 10-18 degrees higher than that which was observed in August of 2021 and 2022.
WHY IS THIS HAPPENING NOW?
Throughout the summer, BHRA has worked hard and continuously with the Bureau of Reclamation (USBR) to supplement warmer-than-average water pulled from the Yellowtail turbines with colder, deeper water drawn from the River Outlet, an opportunity that is not always provided. Because of this effort and collaboration, river temperatures have been kept within acceptable limits for the majority of the hot summer season. However, on Tuesday, September 9th, BHRA was notified by USBR and the Western Area Power Association (WAPA) that the "cold water" supplemental flow provided by the River Outlet needed to be shut off to meet energy-demand needs. Therefore, since Tuesday, all flow has been delivered through the turbine outlets. While the turbines usually deliver cold water, the record-breaking rainfall received in June coupled with the warm air temperatures, has affected stratification of impounded waters. In result, we are seeing warmer water, deeper in the reservoir than normal.
HOW HIGH ARE RIVER TEMPERATURES?
Since Tuesday evening, river temperatures below Afterbay Dam have ranged from between 64 degrees F and 68 degrees F. At the BHRA funded real-time temperature gage at the St X Bridge, temperatures have ranged between 62 degrees F and 70 degrees F. For the past two days, highest water temperatures have been observed between the hours of 3:00 PM and 8:00 PM. You can track real time temperatures at both of these locations by clicking on the RIVER DASHBOARD at the top of the BHRA website.
WHEN WILL RIVER TEMPERATURES DECREASE?
We will start seeing relief by Saturday morning, as USBR gradually opens up the River Outlet to facilitate transmission line work necessary for the hydropower plant. By Monday afternoon through September 15th, all flow will be delivered through the River Outlet (deeper, colder water) as work on the turbines ensues. After this work is complete, water releases will be switched back through the turbines, at which point we will take a good look at conditions and see where we stand. The hope is that cooler nighttime air temperatures, and further settling of the reservoir will take care of water temperature concerns moving forward. However, we will keep you updated on this front.
WHAT ELSE CAN I DO?
In addition to being mindful of where, when and how you fish the Bighorn River during this short time, you can help report sick or dead fish directly to MFWP at this link. Fisheries managers and biologists depend on observations to help them with management decisions.
Thank you for helping Bighorn Wild Trout through this time!