Major weather change brings potential for sediment delivery at Island Park Dam

Updated: Sep 14

A major weather pattern change away from recent record-breaking heat to more seasonable early-autumn conditions brings with it the potential for sediment delivery out of Island Park Dam, albeit a much lower amount for a much shorter period of time than occurred after a pattern change on September 7, 2022.

Export of sediment out of Island Park Dam requires three things to happen:

1. Sediment becomes mobilized and suspended in the reservoir water column

2. Suspended sediment is transported to the dam

3. Sediment is transported out of the reservoir

Here are the factors that can contribute to each of these processes during a summer-to-fall weather change and assessment of potential for those to occur during the change currently underway.

1. Sediment mobilization and suspension. This will almost exclusively occur on the West End of the reservoir.

a. Wave action along the shoreline. Strong winds are expected throughout the afternoon and evening of Thursday, September 8, but those will blow primarily from the southwest toward the dam. This minimizes wave action on most of the exposed shoreline on the West End. Wind is expected to shift to the north northeast Thursday night through Saturday morning. This will increase the potential for wave action, but the wind direction will be primarily perpendicular to the reservoir axis, which minimizes wave size. In 2020, wind direction was from the east northeast, nearly parallel to the reservoir axis, resulting in larger waves. In addition, the reservoir is quite a bit lower than it was at this time in 2020, providing less shoreline area on the West End.

b. Heavy rain on exposed shoreline. Only isolated, fast-moving showers are expected, with little rain actually making it to the ground.

c. Strong vertical mixing in the reservoir due to rapid surface cooling. This is the most likely mechanism we will see over the next 48 hours. Air temperatures are expected to drop by 20 degrees Fahrenheit over a 48-hour period. Reservoir surface temperature is likely to drop by around 5-8 degrees over that time, making the reservoir surface cooler than underlying water, which is very warm for this time of year. This will likely mix the West End, which will mix inorganic and organic material on the reservoir bottom into the water column.

d. Physical displacement of water away from the dam and out onto exposed reservoir-bottom sediment. In 2020, this was a major contributor to sediment mobilization because of very strong east northeast winds parallel to the reservoir axis and reservoir level just high enough that a small change in surface elevation inundated a large area of exposed sediment on the West End. Anticipated wind direction over the next 48 hours is more perpendicular to the reservoir axis than parallel, limiting displacement of water to the West End. In addition, the reservoir is low enough now that a small change in surface elevation will not inundate very much exposed area on the West End.

2. Sediment transport to the dam

a. Rapid cooling of the reservoir surface that results in downward flow of water from the West End toward the dam. This is likely to happen, although cooling will take place over a longer time period than it did in 2020. A 20-degree drop in air temperature happened in 2020 over a 3-5 hour period, whereas this time it is expected to occur over a 36-48 hour period. Slower cooling will result in slower flow toward the dam, allowing sediment time to settle before reaching the dam. Furthermore, water temperature of reservoir inflow from the Henry’s Fork is already quite a bit cooler than that in the reservoir, so even if the West End cools substantially, that flow of cold water may still be warmer than the Henry’s Fork inflow, which is clear. Under this situation, the cooler Henry’s Fork water will largely keep the West End water away from the dam, and the result will be much lower sediment transport to the dam that will be delayed by several days from the weather change itself.

b. In the case of item d above, rapid return of water from the West End back to the dam. As mentioned above, this is not likely to happen because of wind direction and relatively low reservoir content.

3. Sediment transport out of the reservoir

a. Rapid reservoir draft (i.e., more water going out than coming in). Current draft rate is 1,000 ac-ft/day, which is very high for this time of year. This will draw more water out of the reservoir before sediment can settle out behind the dam.

b. High total outflow. Outflow is currently 970 cfs, also very high for this time of year. At a given sediment concentration, high outflow leads to higher total sediment export.

c. High percentage of outflow through the dam gates vs. power plant. About 2/3 of current outflow is being passed through the gates. That allows a direct pathway for sediment reaching the dam to flow out through the gates rather than taking a more indirect path through the power plant.

Our prediction: We will see relatively minor (turbidity increases from 2.5 turbidity units to 5-8 turbidity units), delayed (2-3 days after the weather change—Sunday through Tuesday), short-duration (3-5 days) export of sediment out of Island Park Dam. Even though we think that relatively little sediment will reach the dam, the factors working against us (everything in item 3) will allow this sediment to pass through the dam gates quickly once it gets there.

What has HFF done about potential sediment events like this?

1. Science and monitoring – Doctoral Research Associate, Jack McLaren’s M.S. and Ph.D. research over the past 6 years has provided HFF with a much greater understanding of reservoir dynamics. HFF’s extensive water quality monitoring program, now in its 9th year, has provided real-time turbidity data and the ability to assess how sediment is transported, including how much, when and where that sediment travels. Our annual invertebrate monitoring has allowed us to quantify how aquatic insects respond to sediment deposition and scour in the river.

2. Worked with partners to deliver springtime freshet flows out of Island Park Dam. We know that increased sediment deposition between Last Chance and Pinehaven degrades aquatic invertebrate community structure, but we also know that removal of this sediment via a springtime freshet out of Island Park Dam improves the aquatic invertebrate community. The strong positive response of invertebrate metrics—and hatches experienced by anglers—in 2020 shows that a combination of lower irrigation-season outflow and a high springtime freshet during even a few decent water years can mitigate the negative effects of sediment downstream of Island Park Dam.

3. Minimized reservoir outflow – HFF has developed numerous, critical programs in recent years aimed at conserving water in Island Park Reservoir – including Farms & Fish, Precision Water Management, and Christina Morrisett’s Ph.D. work on the lower Henry’s Fork, among others. Over the previous four years, 2018-2021, HFF programs saved an estimated 20,000 ac-ft of water in the reservoir each year relative to available water supply. This is equivalent to a 100-cfs reduction in summertime outflow.

4. Maximized ability to pass outflow through the power plant – Minimizing summertime outflow from Island Park increases the number of days each year when total outflow stays within the power plant’s capacity. Further, through discussions with power plant stakeholders, HFF initiated the process to lower the power plant’s summertime dissolved oxygen (DO) requirement from 7 mg/L to the Idaho statewide cold-water standard of 6 mg/L. The lower requirement allows the power plant to operate within its license conditions more often in the late summer without the need mix water from the dam gates to meet the DO criterion.

5. UPCOMING: HFF will soon have the capacity to build a computer model of reservoir dynamics that can be used to predict these kinds of events better, plus, investigate feasible options to minimize potential for sediment mobilization.

UPDATE (As of 9/13/22):

Last Thursday, HFF staff saw conditions forming and predicted a sediment event would start Sunday and would increase turbidity into the range of 5-8 turbidity units. View that prediction above. Sediment did in fact start to increase Sunday morning, however, thanks to American Falls Reservoir no longer dropping, flows were able to be decreased from Island Park Reservoir and moved over to the power plant Monday, decreasing turbidity and allowing the event to be short-lived.

Turbidity in the dam outflow began increasing rapidly early on Sunday, and complementary data indicated this increase was all due to suspended sediment. A small increase in production of algae and other phytoplankton occurred on Saturday, indicating local reservoir turnover in the bay where the power plant siphon intake is located. However, Sunday’s turbidity was due to sediment, not phytoplankton production. Turbidity increased from around 3 to 10 turbidity units at the dam gates, as expected. However, turbidity dropped back to around 4 turbidity units yesterday morning when total outflow was reduced and all flow was transferred back to the power plant. Turbidity on the power plant side of the dam is around 5 turbidity units—up from 2.5 prior to last week’s cold front—but still lower than average for this time of year.