The short answer:
A record cold spring, followed by extreme heat and record dry conditions contributed to a need for high outflow from the reservoir for a long period of time, with a lot of water out of the gates side of Island Park Dam. Plus, unexpected rain during month of June added sediment to the system.
We also suspect some sediment mobilized by the Labor Day 2020 wind event still remained near the dam this year because high flows last year and this spring weren't likely long enough to move all of the 2020 sediment away from the dam gates.
NOTE: the entire region has seen high turbidity through late July from flooding and high flows moving sediment that has sat for 10-12 years (i.e., flooding in Yellowstone, dewatering in Madison River last season).
More information:
- Two dam outflow points:
Original Dam Gates: built when the dam was built in the 1930s and sit in the old river channel at the very lowest point in the reservoir.
Power plant siphon: power plant was added to the dam in the 1990s. The siphon is located in a bay off of the old river channel and draws water from about 20 feet up off the bottom of the reservoir.
- Power plant requires a minimum of 200 cfs and reservoir volume of around 35,000 ac-ft (26% full) to operate and its maximum is approximately 960 cfs (+/- depending on reservoir elevation). So, it can operate between ~200 cfs – 960 cfs, but not above or below. If the reservoir drops below around 35,000 ac-ft (26% full), the power plant can’t operate at any outflow.
- During irrigation season, total outflow is set solely for irrigation needs and is sum of gates and power-plant outflow.
- However, during late summer when reservoir water is warm, the power plant’s aeration equipment can’t keep dissolved oxygen high enough to meet its license requirement. When that happens, some or all outflow is transferred to the gates, even when total flow is within the plant’s operating range.
What are the sources of turbidity?
1. Mineral sediment mobilized from reservoir bottom or suspended in the reservoir water column (most common source)
2. Growth of algae and cyanobacteria (aka “blue-green algae) in the reservoir (dominant source during reservoir turnover in spring and fall and also during mid-summer blooms when reservoir conditions deteriorate, e.g. in 2016)
How does turbidity end up in reservoir outflow?
Heavy rain or snowmelt runoff that delivers sediment from tributaries into the reservoir (can happen at any reservoir volume)
Erosion of exposed shoreline sediment by rain (can happen when reservoir gets below around 70% full (95,000 ac-ft).
Mobilization of exposed shoreline sediment by strong wind waves (happens at same elevations as rain erosion)
Mobilization of bottom sediments on the West End by rapid reservoir turnover (rapid cooling of the reservoir surface due to weather changes in the fall; can happen at any reservoir level)
Transport of suspended sediment from the West End by rapid cooling (same mechanism as above in the early autumn)
Erosion of sediment on the bottom of the reservoir by river inflow (happens when reservoir gets below around 35,000 ac-ft (26% full; average end-of-season volume is 44% full)
High outflow through gates (total flow > 960 cfs OR power plant can’t run at full capacity for some reason, e.g., can’t meet dissolved oxygen requirement)
Even more information, including what HFF is doing about this: https://www.henrysfork.org/post/frequently-asked-questions-turbidity-in-island-park-reservoir-outflow