Bighorn Lake and Bighorn River Water Quality Monitoring
Sample and Analysis Plan - 2019
Monitoring water quality in the Bighorn Lake and Bighorn River will provide an understanding of how the lake’s water quality influences the water quality found in the river. The first step to a long-term monitoring program is the development of a Sampling and Analysis Plan (SAP) that answers the following questions:
What factors are responsible for the frequency, extent, duration, and timing of high turbidity in the Bighorn River?
What are the physical, chemical, and biological constituents in the water column that affect aquatic plant and algae growth?
How does Bighorn Lake destratification and Yellowtail Dam flow releases influence water quality in the Bighorn River?
What effects do irrigation withdrawals, irrigation returns, and tributaries have on the water quality in the Bighorn River?
What effect do the Afterbay Dam releases (quantity, timing, and gate) have on total gas saturation
(PSAT) levels in the river from the dam to the Saint Xavier Bridge?
What are the long-term trends in water quality in the Bighorn River?
The water quality of the Bighorn River is primarily determined by Yellowtail Dam releases from Bighorn Lake. The SAP proposes a monitoring site in the lake next to Yellowtail Dam. Field measurements and water samples will be taken along a 400-foot depth profile twice per month from April through October. Below Yellowtail Dam, ten sites will be established on the Bighorn River to fully characterize long-term changes in water quality from the Afterbay Dam to the Yellowstone River confluence. An additional monitoring site will be set up on the Little Bighorn River upstream from where it joins the Bighorn River. Most river sites will be monitored on the same schedule as the lake site. Suspended solids (sediment and algae), nutrients, dissolved oxygen, turbidity, and selenium are the major parameters that will be analyzed.
The water quality monitoring program will begin in April 2020 and continue indefinitely to create a database that documents the Bighorn River’s water quality both spatially (Afterbay Dam to the Yellowstone River – 86 miles) and temporally (seasonally and yearly). It will also provide a foundation for explaining and predicting how the river’s water quality affects other important ecological features (i.e. fisheries, benthic macroinvertebrates, aquatic plant growth, etc.). The SAP will be reviewed periodically to determine if the plan should be revised to more effectively address the study’s objectives.