Characterization of Bighorn River Hydrologic Alterations Below Yellowtail Dam

Lead Scientist

Karin Boyd

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study objectives

The evaluation of flow patterns will help the BHRA understand how the river’s hydrology has changed through time, and how flow management can affect river health. The first study objective is to provide historical context as to how flows on the Bighorn River have been fundamentally altered by dams. The second objective is to look at more recent flow patterns with respect to how the dam is operated, and to consider how operational patterns may affect water quality conditions downstream. And lastly, available information regarding the fishery was compared to flow patterns over the past few decades to consider how trends of trout recruitment and habitat quality may relate to flows. The results are intended to provide a baseline understanding of flow patterns, data availability, and data analysis opportunities so as more information is developed it can easily be given a hydrologic context.

Map showing Bighorn River Watershed above Yellowtail Dam, with contributing watershed areas above Boysen Dam and Buffalo Bill Dam highlighted.

Mean annual hydrographs for Bighorn River near St Xavier showing flow patterns for three timeframes: pre-Boysen Dam (1935-1952), Boysen Dam to Yellowtail Dam(1953-1966), and Yellowtail Dam to present (1967-2019). The comparison shows that Boysen dam strongly impacted Bighorn River flows prior to the completion of Yellowtail Dam.

Number of days mean daily flow exceeded 6,000cfs during incubation windows by year. Results show recent years of high water may have strongly impacted both brown and rainbow trout recruitment rates.

findings

The hydrology of the Bighorn River below Yellowtail Dam has been substantially altered by human development over the past century. Although this development is entirely responsible for creating the blue ribbon tailwater fishery of the river, it has also resulted in complex responses related to river geomorphology, habitat, and water quality. 


Long-term trends show that dam construction has reduced spring flooding while increasing flows in fall and winter. Boysen Dam, which was built between 1947 and 1952, had a major impact on river flows prior to Yellowtail Dam completion in 1966. Once Yellowtail Dam was closed, flows became increasingly simplified downstream, with a lower range in flows resulting in less variability, less flooding, and fewer low flow periods. This effectively created the tailwater fishery we see today. Since then, there have been periods of drought coupled with changes in operating criteria that affected flow patterns on a decadal scale. The core drought years of 2000-2008 occurred before modern operating criteria were established, and this timeframe has a distinct hydrologic signature including persistently low flows and a dominance of flow releases through the dam powerhouse. Since 2009, flows have been consistently higher, resulting in an increased use of the dam spillway supplemental to the powerhouse. Whereas spillway flows are relatively warm, the occasional use of the lowermost river release appears to effectively drop temperatures, especially in late summer.


 Defining optimal flows specifically in support of the tailwater fishery is challenging due to the complexity of processes supporting that fishery. Moderately high flows can rejuvenate side channels but can also scour spawning beds. Low flows can dry out side channels and desiccate redds. Temperature is affected by flow release mechanisms, with river releases from lower in the dam providing cold water pulses to the river. And gas supersaturation appears related to the nature of flow management at Yellowtail and Afterbay Dams, and may also be influenced by water quality in the reservoir. 


implications

The Bighorn River tailwater fishery owes its existence to a highly managed flow scenario and associated dam operations. Whereas the underpinning for the blue-ribbon fishery is the dam, there are aspects of flow management that may contribute to, or detract from, the health of the system. Identifying those relationships will help the BHRA optimize conditions for the fishery while also understanding other process that may indirectly support the fishery or the needs of other stakeholders. With additional water quality sampling and analysis, information will become increasingly available to help the BHRA evaluate cause and effect relationships between natural system hydrology, dam management, and river health.  

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