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FAQ: Turbidity, Sediment, and Island Park Reservoir

Updated: Jun 24



What's the difference between turbidity and sediment?


Turbidity can be:

  1. Mineral sediment mobilized from reservoir bottom or suspended in the reservoir water column

  2. Organic matter - 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)



Is sediment building up behind the dam?


Research indicates sediment is not just building up from inflows to the reservoir. Rather than more sediment coming in, more is being moved from the shallow west end.


What we're learning:

  • River channels, roads and check dams are still visible on the reservoir bottom, suggesting sedimentation rates are not high enough to have buried them

  • During this freshet, turbidity returned to baseline levels within 3 hours

  • If there was a pile of sediment sitting at the dam, it would have continued to exit the dam for the duration of the freshet

  • High turbidity lasts longer during summer events, timing up with cycles like erosion of exposed shoreline (when reservoir level is low) from wind and waves on the West EndGraduate research bathymetric survey in 2017 shows that reservoir volume has not measurably changed in 85 years

  • There have been significant sediment export events like 1992, as well as relatively small amount of natural deposition

Why now and what's next?

Changing climatic conditions (new patterns of increased variability and extremes) seem to be allowing for new, natural sediment to be moved over from the shallow west end of the reservoir. There is much still to learn and options are actively being considered. Stay tuned.


Left: 1,400 cfs mid-summer (8-10 turbidity units) Right: 2,000 cfs, hours after peak April freshet (5-6 units)


What can we do about turbidity (including sediment)?


NOTE: Bolded lines indicate work that is happening now.


  • Keep reservoir as full as possible for as long as possible – water quality benefits (decreased turbidity, including sediment, cooler water in outflow, etc.) and increase trout populations

  • Flow out of power plant side as much as possible – better water quality

  • Changed dissolved oxygen (DO) requirement to statewide standard allows this more often

  • Water Quality monitoring program – millions of data points per year

  • Minimize irrigation season outflow – fishing benefits

  • Freshets – sediment scouring (clearing out) and aquatic insect benefit

  • Infrastructure: variable-depth withdrawal mechanism at gates, heavier-duty aeration equipment at power plant (requires regulatory approval by agencies, $millions to $10s of millions)

  • Physical removal of sediment from reservoir or “dredging”


More information:


How does turbidity end up in reservoir outflow?

  • Mobilization of exposed shoreline sediment by strong wind waves or heavy boat waves (happens at same elevations as rain erosion)

  • Erosion of exposed shoreline sediment by rain (can happen when reservoir gets below around 70% full; 95,000 ac-ft).

  • Heavy rain or snowmelt runoff that delivers sediment from tributaries into the reservoir (can happen at any reservoir volume)

  • High outflow through gates (total flow > 960 cfs OR power plant can’t run at full capacity)

  • Erosion of sediment on the bottom of the reservoir by river inflow (happens when reservoir gets below around 60,000 ac-ft; 44% full = average end-of-season volume)

What are the effects of turbidity?

  • Long-term sediment deposition affects insects on the bottom of the river.

  • Turbidity (from sediment or organic matter) in the water column doesn’t directly harm insects or trout.

    • In fact, it can actually help if part of the turbidity issue is high flows causing turbidity as it moves sediment out of that reach

  • When it comes to fish behavior, here are too many factors at play any given day to correlate a direct link with turbidity.

    • For example, in 2021 it was nearly as turbid as 2022, but enough fish were still rising even when the water was dirty.

What about dredging Island Park Reservoir?


So far, here’s what we know about dredging.


Dredging will not:

  1. Remove all of the sediment from the reservoir. Even if it removes 90%, the 10% that remains will result in as much export into the river if it moves to the dam the next time the wind blows from the NE in fall.

  2. Prevent the reservoir from filling up with sediment again, requiring dredging every few decades.

  3. Prevent sediment from moving to the dam once it enters the reservoir.

  4. Address low DO at the bottom of the reservoir.

  5. Reduce water temperature.

  6. Increase in-reservoir habitat for kokanee and trout.

  7. Affect sediment that is already in the river.

Unfortunately, removing sediment doesn't solve how it got there or how it gets into the river downstream.

Dredging will:

  1. Derail at least 1 season of fishing downstream and set back trout and kokanee populations for at least 1 fish generation (three years).

  2. Require an extended period of 0 flow at Island Park Dam.

  3. Produce an enormous amount of waste to be disposed of.

  4. Require substantial construction of new roads.

  5. Require a federal environmental assessment at a scale unprecedented for this area and could take a decade or more.

  6. Cost 100s of millions of dollars that partners/agencies are not currently interested in funding.

Other considerations:

  • Big projects often come with big, unintended consequences.

  • The risk of something going wrong and having a significant, permanent impact on the river is concerningly high.

These are the factors we are actively considering as we discuss dredging and other solutions to address turbidity below Island Park Dam.


Why is turbidity higher when flow comes out of the gates?

  • Two dam outflow points: 1) original dam gates and 2) power plant siphon

  • Gates were 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 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.

Are there any limitations effecting outflow?

  • Power plant requires a minimum of 200 cfs and reservoir volume of around 35,000 ac-ft (26% full) to operate.

  • Power plant maximum is 960 cfs plus or minus depending on reservoir elevation.

  • During irrigation season, total outflow is set solely for irrigation needs and is sum of gates and power-plant outflow.

Why is dissolved oxygen higher in outflow from the gates?

  • Colder water holds more dissolved oxygen than warmer water, all other factors being equal.

  • DO can be increased by turbulent mixing of atmospheric oxygen into outflow. This happens ONLY on the gates side and ONLY when flow through the gates is at least 200 cfs. In that case, water falling rapidly through concrete tunnel generates enough turbulence (white, foamy appearance on outflow) to keep DO at least 6-7 mg/L on the gates side. There is no turbulence on the power plant side at any flow or operational configuration.

  • NOTE: Injection of air into power-plant outflow using aerators can increase DO on the powerplant side. When the water is cool and DO in the reservoir is not too low, aeration can meet DO requirement. When water warms and reservoir DO is low, aeration can’t maintain the DO requirement.


What are the dissolved oxygen (DO) requirements for rainbow trout?

  • Optimal: 8 mg/L or greater, depending on water temperature and fish life stage (warmer temperature = need for more oxygen, developing eggs/dry = need more oxygen)

  • Lower end of suitable range: 5 mg/L

  • Statewide standard to protect cold-water fish: 6 mg/L or greater

  • Power plant requirements: 8 mg/L during April/May (spawning/rearing), 7 mg/L during March and May (spawning/rearing transition), 6 mg/L over the rest of the year.

  • NOTE: The power plant previously had a constant requirement of 7 mg/L, advocated by HFF and set through the original Federal Energy Regulatory Commission license process in the 1980s. Stakeholders (Fall River Electric, IDEQ, HFF, IDFG and others) agreed to replace this constant requirement with the requirement above a few years ago to 1) provide more protection for sensitive eggs/fry and 2) make it easier for the power plant to meet the requirement during the summer, specifically for the purpose of decreasing turbidity in reservoir outflow.


What can cause algae blooms in the reservoir?

  • Spring/fall turnover (introduces more nutrients into water column)

  • Warm springtime temperatures/early ice-off/rapid warming of reservoir surface

  • Extended hot, sunny weather

  • Excessive reservoir draft

What do typical late-summer conditions look like?

  • Low DO in reservoir at both gates and power plant siphon

  • Total outflow very near power-plant capacity

  • Aerators unable to meet DO requirement if all flow is through power plant

  • Flow is transferred from power plant to gates as needed (at least 200 cfs) to mix turbulent gates outflow (6-7 mg/L) with the best the power plant can do (4-5 mg/L) to meet the state’s 6 mg/L minimum.

  • Turbidity increases as more flow is transferred from power plant to gates.

  • Tradeoffs: gates outflow is colder and has more oxygen (positive for fish) but is more turbid (negative for fishing experience in the short term and for downstream aquatic insects in the long term)

  • Turbidity is even worse when these factors combine with high total outflow, low reservoir elevation, rain/wind, and/or algae blooms.


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