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.

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