Frequently Asked Questions: Turbidity in Island Park Reservoir Outflow



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 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)

  • 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)

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

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.

Short-term solutions

  • Reduce power plant DO requirement. This was already done a few years ago at HFF’s request, based on science and data. The requirement can’t fall any lower than the Idaho statewide cold-water standard of 6 mg/L. This year, had the plant been operating under the old 7 mg/L standard, flow would have been transferred from the power plant to the gates about one week earlier than it did this year.

  • Transfer water back and forth between gates and power plant frequently as conditions change to minimize flow through gates while meeting DO. This is currently being done by the power plant operator.

Long-term solutions

  • Infrastructure: variable-depth withdrawal mechanism at gates, heavier-duty aeration equipment at power plant. Any and all require some degree of regulatory approval by agencies, collaboration among partners, and funding ($millions to $10s of millions, primarily born by fishing stakeholders—there is no requirement for anyone else to do this)

  • Physical removal of sediment from reservoir (would require extensive regulatory review and approval—many years and $millions--plus 10s-100s of millions for the removal itself and remediation of associated negative environmental consequences—again cost born by fishing stakeholders)

  • Minimize irrigation-season outflow

  • Keep reservoir as full as possible all summer (less chance of algae blooms, less exposed shoreline)

  • Deliver springtime high flow through gates (“freshet”) to clean out deposited sediment and keep it in suspension all the way to Ashton Reservoir when aquatic vegetation isn’t there to trap it in the Ranch.

HFF does all of these last 3 things in collaboration with partners through precision water management, modern irrigation infrastructure, and on-farm demand reduction. Paid for by HFF along with some grant funding, most of which requires match from HFF,



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