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Something In The Water?

Updated: Apr 20

NorthWestern Energy (NWE) has been annually conducting aquatic benthic macroinvertebrate (BMI) surveys since 1995 and water quality monitoring reports since 1997 as part of its environmental biomonitoring responsibilities to operate hydroelectric facilities on the Madison and Missouri Rivers (FERC Project 2188). BMI monitoring has been consistently conducted at 11 sites from the Madison River in Yellowstone National Park downstream to near Three Forks and in the Missouri River from Toston downstream to below the Great Falls of the Missouri at Morony Dam. On the Upper Madison, there are 3 sites. One immediately below Hebgen, one near the Kirby Ranch at the West Fork confluence, and one at Ennis.

For water quality monitoring, there are 17 sites on the Madison-Missouri system from Yellowstone to Morony Dam, with 3 sites on the Upper Madison for water quality parameters. Those are: HWY 287 bridge over the Madison just outside of West Yellowstone, just below Hebgen Dam, and Varney bridge. The suite of parameters surveyed at each site is staggering. Everything from arsenic to zinc. The most recent ten year report is 571 pages long…

Below is a graph showing the most recent data on BMI surveys at all sites from 2023.

As you can see, among all 11 sites, Kirby (located near the West Fork Madison) stands out as quite unique in its BMI assemblage. Approximately 66%(!) of all macroinvertebrates sampled at Kirby are non-insects. Ennis contains a mere 5% of non-insects. I requested the appendices to get a closer look at specific relative abundance of individual species and found that of that 66%, two species of freshwater snails (Physella sp. - also known as Bladder snails, and Fossaria sp. - both are native) make up 63% of relative abundance of all macroinvertebrates at Kirby. Another 2 species of snails represent the 3% bringing the total percent relative abundance of snails at Kirby to 66%. That’s a helluva lot of snails. 

When you look closer at the individual species data from Kirby, compared to the other Upper Madison sites below Hebgen and at Ennis, you notice that the insanely high abundance of snails at Kirby is replacing mostly Midges, Caddisflies, and Mayflies in the assemblage (17% relative abundance in all in 2023). Not ideal for fly fishermen and women, and not ideal for stream health.

Fossaria snail: Valley Garden on the Upper Madison

But do trout eat snails? Yep, sure do. Can't imagine that's very comfortable. Some pass through without dying. But what percentage of trout diets in the Upper Madison are snails I don’t think anyone really knows for sure. Regardless, what the heck is that all about? By the time you get to the Ennis site, some 50 plus miles downstream, the relative abundance of snails shrinks to barely on the radar. So trout eat snails, but what do snails eat?

Many snails are omnivorous but feed on large numbers of aquatic plants, and their growth rate is related to the abundance of plants on which they feed. Physella snails in the Upper Madison prefer to live in shallow water, and are rarely found by surveyors in water with any depth. The shallow water is also generally rich in aquatic plants, phytoplankton, and organic matter, which provide abundant food for freshwater snails

Interestingly, many studies reveal water bodies disturbed by human activities (most notably sewage discharge) are more likely to host snails and in larger numbers than undisturbed waters. Could the snails be indicating stream health issues at Kirby?

The short answer is yes. Mean biointegrity scores from 2012-2022 at Kirby meet the criteria for “significant nutrient impairment.” In 2023 Kirby has further decreased in biointegrity from 63% (2022) to below 60% in 2023 (57%). Both Hebgen and Kirby sites were below the long-term average and impairment threshold. Check out this graph from the NWE ten year water quality monitoring report. Between Hebgen and Kirby, something is causing significant impairment. Then by the time you get to Ennis, that impairment disappears or dilutes.

Above the red line is good, below is bad.

What does the water quality report say about Kirby? A similarly concerning trend. Particularly with chlorophyll-a concentrations in the water. Chlorophyll-a is an important index of phytoplankton biomass, and its content could reflect the nutritional status of the water body. Essentially, too much chlorophyll-a is a bad thing. Wadeable streams with chlorophyll-a concentrations greater than 120 mg/sq meter are considered nutrient impaired by the State of Montana. The last year of recorded data available was in 2020. Just below Hebgen near Kirby, scientists recorded a level of 198 mg/sq meter…yikes. But again, by the time you get to Ennis chlorophyll-a concentrations are back down to a normal level. In the chart below, NWE refers to the site below Hebgen as “B2” and the site at Ennis as “B3.”

And as it turns out, phytoplankton play an important role in snails’ diets, tying this all back together. Research shows that chlorophyll-a in water has an important impact on snail occurrence and abundance. A high concentration of chlorophyll-a has been shown to indicate high phytoplankton content in the water column, which provides sufficient food for the growth and development of snails.

Moreover, research also shows us that the occurrence of most snail species is strongly correlated with human disturbance factors such as sewage discharge. Uh oh… there's those words again. Whether or not this is going on upstream of Kirby I can’t confirm, but clearly something is happening upstream leading to downstream issues at Kirby. And if I were a betting man, I'd say it's probably anthropogenic (human source).

All I’m saying is something is negatively affecting the water quality around the Hebgen to Kirby reach based on available evidence, and it's a bit concerning. In summary, Kirby showed nearly 66% of the macroinvertebrate assemblage as snails, and increase from 54% in 2022. Ennis had 5% as snails, an increase from 1% in 2022. Biointegrity scores based on macroinvertebrate rating criteria collected at Kirby indicated “significant impairment.” Ennis showed no such impairment and in fact looks to be in extremely good health. Hebgen to Kirby showed excessively high levels of chlorophyll-a recently. Ennis, no such findings. I’d say “I’m no scientist” but I spent nearly 13 years as a research scientist, where I saw far more subtle relationships turn out to be correlative and this strikes me as more than just a coinkydink. But maybe I'm worried about nothing.

On top of all this, the Hebgen to Kirby reach has had a tough go of it the last few years with the dam failure in November of 2021. Fish population surveys at  Pine Butte (near Kirby) in 2022 revealed Brown Trout abundances were below the 20-year averages for all ages. An interesting quote in the 2022 2188 Fisheries Report by MT FWP says “Age 2 Brown Trout have been below the 20-year average since 2018, indicating other factors may also affect Brown Trout abundance in the Upper Madison River.”

At the end of the day, if we step back and look at the Upper Madison more broadly, it’s in pretty darn good shape overall. Certainly in better shape than neighboring waters. There’s just this one nagging thing happening at Kirby that has my antenna raised. Is it something to be concerned about? Could these trends spread downstream long term? I don’t know. I’m eager to see the forthcoming fisheries report from FWP summarizing 2023 surveys.


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