Photo of salmonfly nymph.


  1. Aquatic macroinvertebrates—insects and other creatures that live in the stream bottom—feed trout and are the basis of fly-fishing on the Henry’s Fork, but they are also important indicators of aquatic habitat quality.
  2. HFF is two years into a long-term program of monitoring aquatic macroinvertebrates at Flatrock, Last Chance, Osborne Bridge, Ashton, and St. Anthony.
  3. Primary conclusions from comparison of 2015 and 2016 data are:
    1. Abundance of macroinvertebrates averages about 47,000 individuals per square meter of stream bottom across all sites.
    2. Mayflies, stoneflies and caddisflies dominate the invertebrate assemblage at Flatrock, Last Chance, and Ashton, but are outnumbered by other organisms at Osborne and St. Anthony.
    3. Aquatic habitat quality ranges from excellent at Flatrock to good at St. Anthony, decreasing with distance downstream from the headwaters.
    4. The only statistically significant differences between 2015 and 2016 occurred at Osborne Bridge, where total abundance of invertebrates decreased—primarily because of a decrease in non-insects—and where habitat-quality index increased, reflecting an increase in percentage of mayflies, stoneflies, and caddisflies.
    5. Most of the mayflies and stoneflies important to fly anglers were found at all five sites. These were pale morning dun, flav, blue-winged olive, green drake, trico, brown drake, and yellow sally.

What is an aquatic “macroinvertebrate?”

As we all learned in high school biology, invertebrates are organisms that lack a vertebral column or “spine”. They include familiar organisms such as insects, worms, crustaceans (crabs, shrimp), and mollusks (clams, snails). Around 97% of all animal species are invertebrates; we vertebrates are definitely in the minority! “Aquatic” invertebrates, then, are invertebrates that live in aquatic environments such as streams and lakes. An aquatic “macroinvertebrate” is one that can be seen with the naked eye. Insects, leeches, worms, snails, and freshwater shrimp are all common aquatic macroinvertebrates that occur in trout streams.

Microscopic photo of freshwater shrimp.

Freshwater shrimp collected from the Henry’s Fork in 2015. Photo by Brett Marshall.

What does “taxa” mean?

I also still remember the mnemonic “Kings Play Checkers On Funny Green Squares” from high school biology as a way to remember the hierarchy of taxonomic classification of organisms: Kingdom, Phylum, Class, Order, Family, Genus, Species. This classification system is the way “scientific” or “Latin” names are assigned to organisms in a precise manner so that scientists always know what organism they are talking about. Common names are not as reliable for identifying organisms because some species have multiple common names (for example Mackinaw and Lake Trout apply to the same species), and some common names apply to multiple types of organisms (for example “perch” refers to members of the perch family but is also used as a common name for some sunfishes). As a reminder of how this works, the table below gives the classification of two important organisms in the Henry’s Fork. Those of you who have been associated with these two organisms for many decades know that taxonomy is fluid—rainbow trout used to be Salmo gairdnerii, and the green drake used to be Ephemerella grandis. Changes in taxonomy occur as new scientific and historical information is discovered.



Rainbow Trout

Green Drake


Animalia (animals)



Chordata (animals with a notochord)

Arthropoda (animals with exoskeleton, segmented body, and jointed appendages)


Actinopterygii (ray-finned fishes)

Insecta (insects)


Salmoniformes (modern Salmonidae)

Ephemeroptera (mayflies)


Salmonidae (Salmon, trout, char, whitefish, grayling)

Ephemerellidae (spiny crawler mayflies)


Oncorhynchus (Pacific salmon & trout)

Drunella (in the Henry’s Fork, includes green drake and flav)


mykiss (Rainbow Trout)

grandis (green drake)


Ideally, all macroinvertebrates would be identified to species, but that is not always possible from samples taken from the stream bottom, given the small size of most individuals and difficulty in distinguishing among closely related species without additional information. Thus, some aquatic organisms are identified to species, others to genus, and others only to family. To be precise, then, we can’t always refer to “species” in results of macroinvertebrate sampling and instead refer to “taxa,” which is the plural of “taxon” and refers to the lowest classification level used in the reporting and analysis of the sample. For example, when we say that a sample contained 32 taxa, that might mean 20 different species, and additional 10 different types identified only to genus, and 2 more identified only to family. In reality, this is at least 32 different species, since each genus and family could be represented by more than one species.

Why monitor macroinvertebrates?

Aquatic macroinvertebrates are the workhorses of aquatic ecosystems. They convert primary energy sources such as plants and algae into trout food, providing the majority of the diets of young trout. Although large trout—especially brown trout—can get a large fraction of their energy from vertebrate prey such as small rodents and fish, in the Henry’s Fork, even adult trout continue to feed primarily on invertebrates. Of course, without aquatic invertebrates, fly fishing would be a completely different activity. Although many popular fly patterns imitate vertebrates and terrestrial insects, the majority of trout fly patterns imitate the various life stages of aquatic invertebrates, primarily mayflies, stoneflies, and caddisflies. As it turns out, these insects are indicators of water quality and overall health of the aquatic ecosystem because most species of mayflies, stoneflies and caddisflies are sensitive to water pollution and habitat degradation. In fact, this group is so important in the assessment of water and habitat quality that it has its own acronym among aquatic ecologists—EPT. This acronym is short for the three taxonomic orders Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies). Higher relative abundance of EPT taxa indicates better water and aquatic habitat quality. Several other quantitative measures calculated from the relative abundance of different taxa complement the EPT percentage to provide indexes of the quality of aquatic habitat. Although HFF maintains an extensive network of water-quality monitoring equipment throughout the watershed, water quality measurements give us data only on the physical and chemical composition of the water itself and not on the quality and quantity of aquatic habitat. Aquatic invertebrates integrate habitat quality and water quality to indicate the overall quality of aquatic ecosystems. Because of this, monitoring of aquatic macroinvertebrates has become the standard method for government agencies, scientists, and organizations like HFF to keep track of trends in aquatic ecosystem health.

Microscopic photo of mayfly nymph.

Mayfly nymph collected at Last Chance in 2015. Photo by Brett Marshall.

What macroinvertebrate monitoring CANNOT tell us

Many aquatic macroinvertebrates, such as leeches, worms, and snails, spend their entire lives in the water. Others, such as most aquatic insects, have both an aquatic and a terrestrial life stage. In fact, the adult stage of common mayflies, stoneflies, and caddisflies provides the most sought-after angling opportunities on the Henry’s Fork—the chance to catch a trout on a dry fly. However, the adult stage of all aquatic insects is very brief compared to the aquatic stage—a few hours to days on land compared with months to years on the stream bottom. As a result, effective sampling of aquatic macroinvertebrates and use of macroinvertebrate measures to tell us about habitat quality relies on sampling the invertebrates while they are in the river—not in the air. That is, aquatic insects are sampled as nymphs or larvae, not as adults. Therefore, the analysis of aquatic macroinvertebrates does NOT tell us anything about a particular hatch of adult insects from a fishing standpoint, especially on any particular day or location in the river. In general, the fishing-related aspects of adult insect hatches are only very loosely related to abundance of the aquatic (nymph) stages and depend on a lot of other factors such as weather, streamflow, fish behavior, and water clarity.


So, I can only describe our aquatic invertebrate sampling and what it tells us about overall aquatic habitat quality; knowing full well that what I report will contradict the personal fishing experience of many anglers. This is because, as explained above, the aquatic invertebrate analysis provides information on aquatic habitat quality, and does NOT reflect hatches of adult insects from a fishing standpoint. This certainly won’t be the first time that what I report from a scientific standpoint is inconsistent with angler experience on the river. More on this in a future blog.

HFF’s Macroinvertebrate Monitoring Program

The previous section described “why” we monitor macroinvertebrates. Now we describe the “who”, “when,” “where,” “how,” and “what.”


HFF staff and interns collect the invertebrate samples, under direction of Brett Marshall, an experienced invertebrate biologist who runs a company called River Contiuum Concepts in Bozeman. Brett has been known as “The Bug Guy” for over two decades and is a national authority on aquatic invertebrates. His consulting firm specializes in assessment of aquatic ecosystems. Brett and his team process the samples and provide the data to HFF. Brett also maintains a “master list” of all aquatic invertebrates HFF and its partners have ever found in the Henry’s Fork Watershed, updating scientific names as necessary to keep pace with advances in identification and taxonomic classification. HFF staff then use statistical methods to analyze the data.


Most aquatic invertebrates have a well-defined life history driven by seasonal patterns in day length, water temperature, streamflow, and other environmental factors. Other than midges, which hatch and reproduce year-round, most aquatic insects in the Henry’s Fork hatch and reproduce during the spring, summer and fall—roughly between the middle of March and early November. In addition, most of the common EPT taxa have a one-year life cycle, meaning that immediately after hatch of a particular insect, that species is represented on the stream bottom only by eggs or very young individuals, which are too small to be sampled and counted. For example, if we sampled in early July, we would be very unlikely to capture any green drakes in our sample, because this year’s cohort would have just hatched, and next year’s are still in the egg stage. Thus, the best time to sample invertebrates on the Henry’s Fork is in the middle of March, immediately prior to the beginning of the spring hatches, when almost all species are at nymph stages that are large enough to be captured and counted. In the first two years of our current sampling program, we collected invertebrates on March 17, 2015 and on March 17, 2016. We will continue to sample at this time each year in order to obtain the most complete sample of all species in a consistent manner.

Photo of Brett Marshall and HFF staff collecting invertebrates from the Henry's Fork.

Brett Marshall, Melissa Muradian, Rob Van Kirk, and Christina Morrisett use Hess samplers to collect invertebrates at Osborne Bridge, March 2016. Photo by James Chandler.


Based on a variety of scientific, logistical, and resource (time and money) constraints, we selected five sites on the mainstem Henry’s Fork at which to conduct annual invertebrate analysis. The five sites are shown on the map below. The Flatrock site is located upstream of Island Park Dam and represents conditions in the headwaters of the river, where effects of dams, diversions, agriculture, and other land uses are minimal. That’s not to say that the Flatrock site is unaffected by land uses upstream and by water management at Henry’s Lake, but cumulative upstream effects of land and water use are minimal at Flatrock compared with points farther downstream. The Last Chance site is in a reach of river that is greatly affected by management of Island Park Dam and is also at a geologic transition, where the river has emerged from Box Canyon but has not yet entered Harriman State Park. Most invertebrate taxa found in Box Canyon and the upper reaches of Harriman are also found at Last Chance. The Osborne Bridge site is representative of the lower Ranch and Harriman East, where the river gradient is lower and substrate is finer (silt, sand, and fine gravel at Osborne versus gravel and cobbles at Last Chance). The Ashton site is immediately upstream of Ashton Reservoir and so is affected by Island Park Dam but also by the inflow of Warm River, which is the most pristine of any of the major tributaries to the Henry’s Fork. Among the sampling five sites, St. Anthony reflects the greatest cumulative effects of upstream uses of the river and its tributaries. The St. Anthony site is downstream of four reservoirs, five hydroelectric power plants, dozens of major highway and county-road bridges, 19 canal diversion points, and hundreds of square miles of agricultural land.

Map of Henry's Fork watershed, showing invertebrate monitoring sites.

Map of Henry’s Fork watershed, showing HFF invertebrate monitoring sites.


Invertebrates are collected using what is called a Hess sampler, which is basically an open aluminum drum that is pushed down into the stream bottom. The substrate on the bottom of the stream is then vigorously stirred to free the invertebrates living there. The drum has a screened opening on one side that allows water to flow into the sampler, and a mesh net across from the opening captures the invertebrates as they are stirred up from the bottom and flow into the net. All large rocks that are present within the area sampled by the drum are manually cleaned with a brush to make sure all invertebrates (especially case-making caddisflies) are scraped into the sampler. The drum has a known area so that the number of invertebrates in the sample can be extrapolated to abundance per square meter of stream bottom. We collect multiple samples at each site to account for variability across the stream bottom. In 2015, we collected three samples per site; in 2016 we collected five per site. A larger number of samples reduces uncertainty and increases statistical power to detect changes across years and sites.

Photo of a sampled cobble with caddisfy cases and a golden stonefly nymph

A brush is used to scrape invertebrates off of rocks that are enclosed by the sampler. This rock was in a sample from the Ashton site and has caddisfly larvae and a golden stonefly nymph on it. Photo by James Chandler.

Each sample is emptied out of the net and into a plastic jar and then preserved with alcohol. At Brett’s lab, the sample is cleaned and sorted, to separate the invertebrates from sand, gravel, and plant material. Individual invertebrates are then identified and counted. In samples from the Henry’s Fork, which contain very large numbers individuals, only a subsample of each full sample is used for counting and identification. Brett aims for identification of about 200 individual invertebrates from each sample, and he uses a strict quality-control procedure. After a sample is processed, a second technician sorts the sample to validate the result of the first technician. If there are discrepancies, a third person examines the sample. As a result, Brett’s lab is known for providing the highest level of accuracy and precision in quantitative analysis of invertebrates; he consistently finds more small organisms in samples than is the standard in the industry.

Photo of Melissa pouring preservative into a jar containing the invertebrate sample.

Melissa and Rob preserve a sample of inverterates. Photo by James Chandler.


Brett’s lab reports raw data, which consists of the number of each identified taxon present in each sample. He also reports summary data such as total number of individuals per sample, number of different taxa, number individuals in the sample from the EPT taxa, and various indicator metrics. We have focused on four different metrics:

  1. Abundance (number of individuals per square meter of stream bottom)
  2. Shannon’s diversity index (higher diversity means more individuals spread across a larger number of different taxa)
  3. Percent EPT (fraction of total number of individuals that are mayflies, stoneflies, and caddisflies), and
  4. Hilsenhoff Biotic Index (a measure of habitat and water quality)

We statistically analyze the data Brett provides us to look for trends across years and sites.

What did we learn?

In general, abundance of macroinvertebrates is high in the Henry’s Fork, averaging 47,000 individuals per square meter across all sites in both years. The percentage of these individuals belonging to the EPT taxa averaged 50% across sites and years, meaning that a typical square meter of stream bottom on the Henry’s Fork is home to around 23,500 individual mayflies, stoneflies, and caddisflies. Although the mean abundance of 47,000 individuals per square meter is not unusually high for rivers in the Greater Yellowstone Region, very high abundances often occur when invertebrate assemblages are dominated by very large numbers of small-bodied organisms such as midges and New Zealand mud snails, reflecting degraded habitat conditions or nonnative species invasions. At most sites in the Henry’s Fork, high abundances of invertebrates are accompanied by high fractions of mayflies, stoneflies and caddisflies and not by high abundances of organisms that indicate habitat degradation or invasions of nuisance speices. Therefore, our quantitative data corroborate what anglers have known for decades—the Henry’s Fork is a unique trout stream because it is home to very large numbers of mayflies, stoneflies, and caddisflies.

In the Henry’s Fork, the high abundance of individual organisms is spread across a large and diverse set of taxa—again, dominated at most sites by mayflies, stoneflies, and caddisflies. The mean number of different taxa found at each site ranged from 27 to 39. Large numbers of relatively large-bodied mayflies, stoneflies, and caddisflies were present at most sites. For example, at Flatrock in 2015, 56% of the individual invertebrates present (about 31,000 individuals per square meter of stream bottom) were pale morning duns (Ephemerella excrucians), flavs (Drunella flavilenea), green drakes (Drunella grandis), and brown drakes (Ephemera simulans), a phenomenal number of these important mayfly species! Speaking of important insects, the most widespread of the important mayflies and stoneflies were pale morning duns and flavs, which were present at all five sites in both years. Five more important mayflies and stoneflies were found at all five sites in at least one year: green drakes, brown drakes, blue-winged olives (Baetis), tricos (Tricorythodes) and yellow sally stoneflies (Isoperla). The giant salmonfly (Pteronarcys californica) was found at all sites except Flatrock, the mahogany dun (Paraleptophlebia heteronea) was found at all sites except Osborne Bridge, and the March brown (Rithrogena morrisoni) was found at all sites except Ashton. Of course, various species of midges and caddisflies, important trout food and fly-fishing insects in their own right, were found at all sites in both years.


Abundance varied substantially from sample to sample, resulting in high statistical uncertainty in comparisons involving abundance. This is reflected in the wide comparison intervals depicted in the figure below. Nonetheless, the graph shows that there were no systematic trends in abundance either across sites or between years. Mean abundance decreased slightly between years at Flatrock and Last Chance and increased slightly between years at Ashton and St. Anthony. However, none of these small differences were statistically significant. The large decrease in abundance at Osborne Bridge between 2015 and 2016 was statistically significant; mean abundance there decreased from 86,500 individuals per square meter in 2015 to 39,000 individuals per square meter in 2016. Closer inspection of the data showed that this decrease was due primarily to a decrease in the number of midges and of non-insects such as worms. More on this below.

Graph showing abundance of invertebrates at each site in 2015 and 2016.

Abundance of invertebrates at five sites on the Henry’s Fork in March 2015 and March 2016. Points show the average over all samples taken in the particular year at the particular site. The bars are called “comparison intervals” and depict statistical comparison across sites and years. Bars that overlap indicate no statistical difference at 95% statistical confidence. Bars that do no overlap indicate a difference at 95% confidence.


We used Shannon’s diversity index, which is a standard and widely used metric in ecology. Without going into the mathematical details, Shannon’s diversity is 0 if all individuals belong to the same taxon. As the number of different taxa increases, the diversity index increases. The maximum diversity index is achieved when there are equal numbers of all taxa present, and this number continues to increase as the number of taxa increases. In our Henry’s Fork macroinvertebrate samples, the number of different taxa averaged 32 per site. With 32 taxa present, the maximum possible diversity index is 3.5. Thus, the vertical scale in the figure below ranges from 0 to 3.5, representing the full range of possible values across our sites.

Graph showing Shannon's diversity at each site in 2015 and 2016.

Diversity of invertebrates at five sites on the Henry’s Fork in March 2015 and March 2016. Points show the average over all samples taken in the particular year at the particular site. The bars are called “comparison intervals” and depict statistical comparison across sites and years. Bars that overlap indicate no statistical difference at 95% statistical confidence. Bars that do no overlap indicate difference at 95% confidence.

It is easy to see from the graph that there is a statistically significant, systematic increase in diversity with distance downstream from the headwaters. The diversity increases with distance downstream because both aquatic productivity and habitat diversity increase with distance downstream from Flatrock. At Flatrock, the stream bottom is relatively homogeneous, consisting of coarse gravel, with a few larger cobbles here and there. Productivity is limited by a relatively short growing season, cool water temperatures, low nutrient input (as measured by our water-quality monitoring), and a relatively simple food web. In other words, diverse habitat types and food sources do not exist to support large numbers of different kinds of organisms at the Flatrock site. With distance downstream, water temperatures increase, the growing season becomes longer, more nutrients are available, aquatic vegetation becomes more abundant, and the aquatic food web becomes more diverse. These conditions all contribute to the ability of the stream to support larger numbers of more different types of invertebrates. At the St. Anthony site, where diversity is highest, the stream bottom consists of a variety of particle sizes ranging from fine silt to cobbles. Large logs and roots from cottonwood and other trees that grow in the floodplain and fall into the river provide additional types of aquatic habitat. All of these conditions increase the number of different types of organisms that are present and the abundance of each type at St. Anthony. The graph also shows that there were no changes in diversity between years—some sites were up slightly, while others were down slightly, but none of these differences between years were statistically significant.

Percent EPT

This metric is simply the fraction of total individuals at the site that are mayflies, stoneflies, and caddisflies. The figure below shows a statistically significant, systematic decrease in percent EPT with distance downstream from Flatrock. Averaged over the whole watershed and both years, percent EPT was 50%, ranging from a high of 75% at Flatrock in 2016 to a low of 22% at St. Anthony in 2016. Notice that the systematic decrease in EPT with distance downstream mirrors the systematic increase in diversity. This indicates that the increase in diversity is due to increases in the relative abundance of non-EPT taxa such as midges and worms. Generally, large numbers of midges and worms indicate an increase in the presence of fine sediment on the stream bottom. The Osborne and St. Anthony sites have the largest amount of fine sediment of all of the sites, and this is reflected in the percent EPT. There were no statistically significant differences in percent EPT between years; as with diversity, some sites increased between years while others decreased.

Graph showing percent EPT at each site in 2015 and 2016.

Percent mayflies, stoneflies, and caddisflies at five sites on the Henry’s Fork in March 2015 and March 2016. Points show the average over all samples taken in the particular year at the particular site. The bars are called “comparison intervals” and depict statistical comparison across sites and years. Bars that overlap indicate no statistical difference at 95% statistical confidence. Bars that do no overlap indicate difference at 95% confidence.

Biotic Index

The Hilsenhoff biotic index (HBI) is based on the tolerance of the various invertebrate taxa to pollution and habitat degradation. All taxa are given a score from 0 (intolerant of degradation) to 10 (tolerant of degradation), and the HBI is an average of these tolerance scores across the organisms in the sample. Because intolerant taxa will disappear and be replaced with tolerant taxa when water quality and habitat are degraded, lower scores indicate better overall aquatic habitat conditions. The figure below has the vertical axis reversed so that the higher the point on the graph, the better the habitat conditions. A qualitative key to the numeric scores is indicated by the colors on the graph.

Graph showing HBI scores at each site in 2015 and 2016.

Biotic index (a measure of overall water and habitat quality) at five sites on the Henry’s Fork in March 2015 and March 2016. Points show the average over all samples taken in the particular year at the particular site. The bars are called “comparison intervals” and depict statistical comparison across sites and years. Bars that overlap indicate no statistical difference at 95% statistical confidence. Bars that do no overlap indicate difference at 95% confidence.

The most obvious observation from this graph is that overall health of the aquatic ecosystem decreases systematically with distance downstream of Flatrock, reflecting the spatial trends apparent in the previous two graphs—specifically that diversity increases and percent EPT decreases with distance downstream. This should come as no surprise to any of us, given that the effects of water and land uses are cumulative and increase with distance downstream. We expect the most pristine conditions at the headwaters and the most degraded conditions at St. Anthony. However, there is a lot more information in the HBI graph than simply the general decline in conditions as we move down the watershed, and these deserve some more careful discussion, given below.

Additional observations

  1. Notice how closely the HBI scores match percent EPT. The relative positions of the HBI scores across sites and years are the same as those of percent EPT, emphasizing the utility of mayflies, stoneflies, and caddisflies as indicators of overall aquatic ecosystem health.
  2. Except for the Osborne site in 2015, all of the HBI scores—even at St. Anthony—fall into the “good,” “very good”, or “excellent” classes. The 2016 HBI score at St. Anthony was right on the border between “fair” and “good” but still technically fell in the “good” range.
  3. The strongest statistically significant change between 2015 and 2016 observed across any of the metrics was the substantial improvement in the HBI score at Osborne Bridge between 2015 and 2016. The 2015 score was solidly in the “fair” range, whereas the 2016 score fell at the top end of the “good” range. This improvement in HBI at Osborne was accompanied by a large decrease in abundance and by slight increases in diversity and percent EPT. Taken together, these indicate that between 2015 and 2016, the Osborne site lost a large number of tolerant taxa such as midges and worms, resulting in higher EPT and better HBI. In addition, diversity went up slightly at Osborne, indicating that in 2016, the individuals present were more uniformly distributed across the taxa than in 2015, when the invertebrate numbers were dominated by fewer taxa—in that case taxa that were more tolerant of habitat degradation. What could have caused this substantial improvement in conditions at Osborne Bridge? This is highly speculative, of course, but it could have been the very high flow delivered from Island Park Dam during late July of 2015 for the Chester hydroelectric project test. This high flow could have scoured some fine sediment from the Osborne site, improving habitat conditions there. Recall that there was another high-flow event in the spring of 2016, but that event occurred after we had already done the 2016 sampling. If we see continued improvement in 2017 at Osborne, that will give us more evidence that the short high-flow events may be helping to move fine sediment out of the Osborne Bridge area.
  4. Habitat quality at Last Chance was right on the border between “very good” and “excellent” and did not change between years. Percent EPT was not statistically different at Last Chance between 2015 and 2016, averaging over 60% across years. These observations indicate that overall habitat quality and composition of the invertebrate community at Last Chance was not negatively affected by very low flow out of Island Park Dam during the winter of 2015-2016. In fact, I was surprised that overall habitat quality and percent EPT are only slightly lower at Last Chance than at Flatrock, despite what we know to be major effects on streamflow and water chemistry caused by Island Park Reservoir and its operation.
  5. Percent EPT and HBI were higher at Ashton than at Osborne Bridge, despite being farther downstream in the watershed. This is because the unregulated, high-quality water flowing into the river between Pinehaven and Ashton (primarily from Warm River) greatly reduces the effects of Island Park Dam on water quality and water quantity. However, the declines in percent EPT and habitat quality that occur in the relatively short distance between Ashton and St. Anthony reflect the greatly increased intensity of land and water use in that reach of river.

Microscopic photo of midge larvae.

Midge larvae collected from the Henry’s Fork in 2015. Photo by Brett Marshall.

What’s next?

We will continue the same sampling and analysis in 2017, with particular emphasis on detecting any changes that may have resulted from extended periods of high flow and increased turbidity below Island Park Dam during the summer of 2016. In addition, we plan to dig through all invertebrate data we have in our files, going back at least to the mid-1990s, to see if we can detect any long-term changes in macroinvertebrate communities.