Why Have Insect Hatches Improved so Much in 2020?

Photo of rainbow trout
  • Percent mayflies, stoneflies, and caddisflies in invertebrate samples at Osborne Bridge averaged 57% in 2019, compared with 41% during and after the recent drought and 17% in 1993.
  • A high springtime freshet and low irrigation-season outflow from Island Park Reservoir, as occurred in 2018-2020, accelerates transport of sediment out of the Harriman reach, providing an explanation for improvement in hatches since 2017.
  • In the long run, net sediment transport out of the river between Island Park Dam and Pinehaven is around 1,600 tons per year. At that rate, 45-90% of the sediment deposited during the 1992 event has been removed to date, providing an explanation for substantial improvement since 1993.
  • HFF will continue to work with the Henry’s Fork Drought Management Planning Committee to implement the high springtime/low summertime flow regime that appears to be beneficial for aquatic insects. 

Following three good years of winter flow, we all expected to see large numbers of 3-year old fish (14-18 inches long) in the river between Island Park Dam and Riverside campground this summer. Because of covid-19 restrictions this spring, Idaho Department of Fish and Game was unable to perform its annual trout population survey in Box Canyon. However, this spring HFF counted the second-highest number of spawning-sized rainbow trout in the Buffalo River fish ladder since the ladder was built in 2005. Using correlation between Buffalo River spawner numbers and the Box Canyon trout population, we estimate that the rainbow trout population this year is around the same is it was in 2019, which was the fourth highest in a record that goes back to 1978. Reports from anglers overwhelmingly indicate large numbers of 14-18 inch trout in the river this year. No surprise.

What was less expected this year was the dramatic improvement in aquatic insect hatches observed and reported by many anglers who have fished the Harriman Ranch and adjacent river reaches regularly for decades. I have heard reports that some hatches have been as good or better than they were in the 1980s and certainly better than anything seen in recent years. Personally, I have fished the Ranch since 1977, and I have been fishing it regularly this year. My experience corroborates what I have heard from other anglers. Good hatches + a large number of fish = outstanding fishing.

But as a scientist, I want to see the data that support observations of anglers (including myself, as an angler) and find plausible mechanistic explanations for the dramatic improvement in aquatic insect hatches this year.

Invertebrate Data and Methods

HFF’s modern invertebrate monitoring program began in 2015, with standardized, replicated, stream-bottom (“benthic”) sampling at five locations in the watershed: Flat Rock, Last Chance, Osborne Bridge, Ashton (upstream of Ashton Reservoir), and St. Anthony. See my earlier blog for a description of methods, locations, and results from the first two years of the program. In 2019, we added sites on the South Fork Snake River, and you can read about data from the South Fork in Bryce’s blog.


To help answer the question posed above, we used data from the Osborne Bridge location, which is the one of our five sites most susceptible to effects of deposition and scour of fine sediment related to operation and management of Island Park Dam. We have not yet processed the 2020 samples, but we have data from 2015-2019. As it turns out, researchers at Idaho State University sampled invertebrates at Osborne Bridge in the early spring of 1993, after the 1992 drawdown of Island Park Reservoir. That event exported an estimated 50,000-100,000 tons of fine sediment out of the reservoir and into the river downstream. That gave us a sixth—and very important—year of data.


As for the metrics (quantitative measures calculated from the raw data), we chose two that are most relevant to both anglers and assessment of the overall integrity of the aquatic ecosystem. The first metric is called the Hilsenhoff Biotic Index (HBI), which reflects the overall condition of the aquatic ecosystem. It is a weighted average of tolerance of each species or other taxonomic group to habitat degradation and organic pollution. Tolerance is rated on a 1-10 scale, with 1 being the most sensitive species (least tolerant) and 10 being the most tolerant of degraded conditions. So, a lower HBI score indicates that the invertebrates present are very sensitive to habitat degradation, indicating good habitat conditions. High HBI scores indicate degraded habitat.

The second metric is simply the percentage of individuals present that are mayflies, stoneflies and caddisflies. We refer to this metric as “Percent EPT,” where the EPT stands for the three taxonomic orders Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies).

Importance of Time of Year of Sampling

The Idaho State University data are comparable to ours not only because the same location and sampling methods were used, but also because their samples were collected at the same time of year as ours—late winter/early spring. This is critical to comparisons, because all species of invertebrates present in the river at that time of year are large enough to be collected in samples. Once the first hatches of the spring start to occur, nymphs of species that have just emerged are not yet large enough to be sampled. As a simple example, consider Green Drakes, a species that hatches over a relative short time period and has a one-year life cycle. In mid-July, just after they are finished hatching, only eggs and very tiny Green Drake nymphs are present in the river and would not be collected in the sampler if we sampled at that time of year. Unfortunately, most other historic invertebrate data sets available for the Henry’s Fork were collected during late spring, summer, and fall, reducing the comparability with the late winter/early spring data we collect now.

The invertebrates present in the river in mid-March, when we sample, reflect habitat conditions in the river since those individuals were first deposited as eggs the previous spring, summer or fall. That time period includes two major streamflow events that affect river substrate (the stream bottom) in which the invertebrates live. The first is the springtime freshet (peak flow), or lack thereof in the regulated system downstream of Island Park Dam. The second is outflow from Island Park Reservoir during irrigation season. These two flow characteristics determine the amount of sediment scour and deposition from the stream bottom.

Results: Invertebrate Metrics Have Improved over Time

Before providing more details on the sediment part of the equation, let’s just jump right to the invertebrate metrics. HBI averaged 3.8 (“very good”) in 2019, compared with 4.8 (“good”) during and after the 2013-2016 drought (samples from 2015-2018) and 6.1 (“fair”) in 1993. Percent EPT averaged 57% in 2019, compared with 41% in 2015-2018 and 17% in 1993. These two metrics show that the invertebrate community has improved modestly since the 2013-2016 drought and substantially since the 1993 sediment event.

Possible Explanation: Sediment Transport and Deposition

Our research and monitoring over the past seven years, some of it published in peer-reviewed journals, has shown that most transport of fine sediment out of Island Park Reservoir occurs from mid-July through the end of October, a time period when reservoir outflow is high and/or reservoir elevation is low. Once aquatic vegetation becomes dense in the river reach between Last Chance and Pinehaven, most of the sediment delivered from the reservoir is trapped by macrophytes (aquatic vegetation) and remains in the stream channel through late summer, fall, and winter. Aquatic vegetation is eaten by waterfowl and senesces over the winter, leaving sediment exposed on the stream bottom. When streamflow increases in the spring, this sediment is then mobilized and transported out of the reach.

Annual Sediment Budget

Our instrument network allows us to calculate sediment loads at Island Park Dam, in the Buffalo River near its confluence with the Henry’s Fork, and at Pinehaven. Because sediment inputs from sources between the Buffalo River confluence and Pinehaven are minimal, our estimate of sediment inflow into the reach between Island Park Dam and Pinehaven is the sum of sediment loads from the reservoir and the Buffalo River. On average, sediment is removed from the Henry’s Fork between Island Park Dam and Pinehaven during the spring and early summer, is deposited in that reach from mid-summer to early fall, and is roughly constant through the winter.

Graph of sediment budget between IP Dam and Pinehaven

Average sediment budget for the Henry’s Fork between Island Park Dam and Pinehaven. Suspended sediment data are averaged over 2014-2019 for Island Park and Pinehaven and over 2016-2019 for the Buffalo River. Streamflow data are averaged over 2006-2019.

Long-term Sediment Budget

Since 2006, late-summer deposition of fine sediment has averaged 400 tons/year, and average spring scour of sediment has averaged 2,000 tons/year, for a net removal rate of 1,600 tons/year. For perspective, the 1992 sediment event deposited between 50,000 and 100,000 tons in the Henry’s Fork between Island Park Dam and Pinehaven. At an average removal rate of 1,600 tons/year, roughly 30-60 years will be required to remove all of the sediment deposited during that event. Looked at another way, roughly 45-90% of the 1992 sediment has been removed from the river to date.

Relating Sediment and Invertebrates

Because we do not have sufficient water quality data to calculate sediment loads directly to pair with each of the invertebrate samples, we used a surrogate streamflow measure to estimate net sediment scour/deposition. This streamflow measure is mean springtime (March 16 – May 31) outflow from Island Park Dam divided by late-summer through early fall (July 15 – October 31) outflow. This “spring:late summer flow ratio” should indicate springtime scour relative to subsequent summer sediment deposition and thus the habitat conditions invertebrates experience between when their eggs are laid in the spring/summer/fall of a given year and when we sample them the following March.

Results: Invertebrate Metrics Reflect Sediment Regime

Both HBI and percent EPT and show statistically significant relationships to the spring:late-summer flow ratio, with higher ratios leading to higher percent EPT and lower HBI scores, indicating better habitat quality. Of the six years of data, invertebrate metrics were poorest in 1993, immediately after the 1992 sediment event, and highest in 2019, reflecting the large unmanaged freshet during May 2018 that resulted from a large rain event when the reservoir was already full. Although sample size is small, these results suggest a positive response to net transport of sediment out of the river reach between Island Park Dam and Pinehaven and a mechanism that could explain why hatches have improved noticeably over the past two years since the effects of the 2013-2016 drought have waned.

Graph of HBI versus flow ratio

Hilsenhoff Biotic Index (HBI) at Osborne Bridge as a function of ratio of spring (March 16 – May 31) to late-summer (July 15 – October 31) streamflow at Island Park Dam. Higher values of this ratio indicate higher springtime flows and lower irrigation-season flows.

Graph of percent EPT versus flow ratio

Percentage of EPT taxa (mayflies, stoneflies and caddisflies) at Osborne Bridge as a function of ratio of spring (March 16 – May 31) to late-summer (July 15 – October 31) streamflow at Island Park Dam. Higher values of this ratio indicate higher springtime flows and lower irrigation-season flows.

Keeping Water in Island Park Reservoir Benefits Insects, too

Based on the data presented above, streamflow management at Island Park Dam is critical to maintaining the high-quality invertebrate community we currently have in the river. In particular, high springtime flows followed by relatively low irrigation-season flows will continue to scour sediment remaining from the 1992 event in the long run and maintain high-quality substrate from year to year. HFF’s programs to limit draft of Island Park Reservoir were initially aimed at improving winter flow to benefit trout recruitment, but we now know that limiting draft of Island Park Reservoir also improves water quality and fish survival in the reservoir, as well as water quality and fishing experience downstream of the reservoir.

This analysis adds more benefits of limiting draft of the reservoir to that list. Lower draft necessarily means lower outflow during irrigation season, which leads to lower sediment export out of the reservoir. It also means that the reservoir enters the spring nearly full, so that high flows during the spring (the freshet) can be delivered without jeopardizing reservoir fill. In 2018, a spring rainstorm occurred when the reservoir was already full, resulting in a natural freshet flow that was passed through the full reservoir. In 2019, good inflows and high reservoir levels allowed a three-day managed freshet, which was discussed and planned ahead of time with members of the Henry’s Fork Drought Management Planning Committee.

2020 Freshet Operation

In 2020, the freshet was sort of a hybrid of a managed and natural freshet, initially planned for April 29 – May 1 as a managed freshet after discussions with Fremont-Madison Irrigation District, U.S. Bureau of Reclamation and other Drought Management Planning Committee members. However, because of high inflow due to rapid snowmelt throughout the operation and heavy precipitation on April 30, the managed freshet ended up essentially delivering the river’s natural flow. In fact, reservoir operations between early December and early May generally passed inflow through the reservoir, which stayed in the range of 116,000 – 120,000 ac-ft during that time period. Ice cover remained on the reservoir through May 3, so the reservoir could not have been filled much beyond its actual content during the freshet delivery anyway, to avoid ice from reaching the spillway infrastructure.

Graph of natural vs. regulated flow at Island Park

Graph of natural and regulated flow at Island Park for water year 2020 to date, showing net change in reservoir storage. Spike around May 1 is the 2020 freshet.

Graph of Island Park Reservoir volume

Graph of Island Park Reservoir volume for water year 2020 to date.

Between April 15 and May 03, an estimated 555 tons of sediment (around 25% of mean annual scour) was transported out of the river reach between Island Park Dam and Pinehaven. Prior to the first outflow increase of the spring, sediment scour was essentially zero. Each of the three subsequent outflow increases resulted in increased sediment transport out of the target river reach. The freshet operation itself had the greatest effect on sediment removal during the first 48 hours, after which time sediment transport decreased. This suggests that with a relatively large and abrupt increase in streamflow, a 2-3 day freshet can accomplish a large amount of scour with minimal reservoir draft.

Graph of sediment transport during 2020 freshet

Graph of sediment budget between Island Park Dam and Pinehaven, April 15 - May 3 2020.

Future Management

Given the apparent success in improving invertebrate communities with the combination of a high springtime freshet and relatively low irrigation-season flows, HFF will continue to work with the Henry’s Fork Drought Management Planning Committee to meet these flow-management objectives to the greatest degree possible in a given water year. We will also continue our collaborative programs with irrigation entities and individual farmers to reduce irrigation demand and improve irrigation-system management. And of course, we will continue to collect data, refine our understanding of the aquatic system, and provide the best science possible to inform water management decisions.