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High Turbidity Headlines 2020 Water Quality

Updated: Jan 12, 2022

  • Temperatures were below average and highly variable during the spring but above average for most of August and September, reflecting air temperature. Daily means were below stressful levels for trout except for a few days in late July and early August. Maximum instantaneous temperatures recorded were 73.5 degrees at Pinehaven and 75.7 degrees at St. Anthony, both on August 2.

  • Dissolved oxygen followed typical seasonal patterns, and daily means were well above stressful levels for trout, except at Island Park Dam, where late-summer values were very close to but still higher than the power plant’s requirement.

  • Phytoplankton and cyanobacteria production also followed typical seasonal patterns and was highest during the spring when sunlight and nutrients were first available. As expected, production was highest between Island Park Dam and Pinehaven.

  • Turbidity also followed seasonal patterns and was generally highest during spring runoff and following a few heavy rain events. Turbidity between Island Park Dam and Pinehaven was generally higher than the 2014-2019 average and the highest since 2016, as reservoir outflow and draft were higher this year than they have been since 2016. High turbidity during September is the longest-duration, highest-magnitude event we have recorded and was probably the highest since the 1992 sediment event.

We maintain a network of 13 permanent water-quality monitoring instruments called “sondes.” These instruments record water temperature, dissolved oxygen, conductivity, two measures of phytoplankton production (algae and similar organisms), turbidity, and water depth every 15 minutes. In addition, we deploy a 14th “roving” sonde at temporary locations for short periods of time to address specific scientific questions or monitor specific activities along the river. Ten of the permanent sondes are located on the Henry’s Fork and its tributaries, and three are located on the South Fork Snake River. The roving sonde is currently deployed upstream of Ora Bridge, as a control site to monitor any water-quality effects of construction of Ora Bridge. Most of the sondes are deployed only during the spring, summer and fall, although four of the permanent sondes remain in the river all winter. For a map of sonde locations and complete description of the sonde network, see our water quality website.

We also partner with Idaho Department of Environmental Quality to monitor water quality in Island Park Reservoir with a sonde lowered to the bottom of the reservoir in two locations once each week. In 2018-2019, we partnered with Idaho Department of Fish and Game to monitor water quality in Henry’s Lake using the same type of sonde profiles.

In addition to the sonde network, we collect weekly water samples that are analyzed for phosphorus, nitrogen, turbidity, and suspended sediment. The suspended sediment concentrations are paired with the turbidity measurements to develop a statistical relationship between the two measures. This allows us to use turbidity data collected continuously by the sondes to calculate suspended sediment concentration in the river. When concentration is multiplied by streamflow, the result is total sediment load. Thus, the sonde network allows full accounting of suspended sediment through the river, including where it originates and where it is deposited. We also collect annual samples of aquatic macroinvertebrates, which, when paired with other water quality data, give us a complete assessment of overall aquatic ecosystem “health” in different river reaches. Lastly, graduate research conducted by Jack McLaren, both in 2016-2017 for his master’s degree and now for his Ph.D., is answering fundamental scientific questions about the structure and function of aquatic ecosystems in the upper watershed and how those are affected by Island Park Reservoir.

With such a large program that generates so much data, a summary of water quality would require much more space even than I devote to hydrology and water management. I have already posted a blog earlier this year summarizing one of the most important results of our macroinvertebrate data through 2019, and we have not yet analyzed samples from 2020. So, I’ll not report on macroinvertebrates here. The water-year 2020 summary will focus on the water quality parameters most important to anglers—temperature, dissolved oxygen, phytoplankton, turbidity, and suspended sediment—at our Henry’s Fork sonde locations of most importance: Flat Rock, Island Park Dam, Pinehaven, Marysville (just upstream of Ashton Reservoir), Ashton Dam, and St. Anthony. This blog overs all but suspended sediment, which deserves its own separate treatment. Because most of the sondes are deployed only spring-fall and because water quality is constant over the winter (cold, with low turbidity and biological activity), I summarize water quality from April 1 through the end of the 2020 water year.

Water Temperature

In general, water temperature is determined primarily by air temperature at all of our sonde locations except Island Park Dam. Temperature there is determined largely by reservoir physics, so temperature changes over the seasons of the year but not very much from day to day. As detailed in my water year 2020 summary blog, air temperature was near the long-term average during April-June but highly variable. Water temperature at all sites except Island Park reflected this. However, note that because our water quality data date back only to 2014, temperatures over our water-quality period of record are a degree or two warmer than averages over the longer 1989-2019 record I use for climate data. So, for example, you can see the highly variable nature of water temperatures at Flat Rock during the spring but also notice that overall, water temperatures were below our 2014-2019 average, which would be roughly equivalent to the 1989-2019 average. This same pattern is apparent at all of the other sites except Island Park. However, even at Island Park, water temperature was generally below average throughout the spring and early summer, although day-to-day variability was very small, typical of water temperature in outflow from large reservoirs. More “average” temperatures in 2019 and 2020 were noticed by anglers in the form of more “normal” timing of various insect hatches, compared to that of other recent years, which were much warmer than the 1989-2019 average.

Air temperatures during early July were near average and very stable, due to an extended period of dry weather that started at the beginning of the month. Correspondingly, water temperatures followed the same pattern. However, by late July, above-average air temperatures set in and remained in place more or less for the remainder of the summer. Water temperatures were also above average from late July through the end of the water year, with the exception of a few days in September following strong cold fronts. Peak water temperatures for the year occurred in late July and early August. During that time, daily average water temperatures exceeded 67 degrees F for several days at Pinehaven and Ashton Dam and for over a week at St. Anthony. Although temperature requirements for trout vary across species, 67 degrees is roughly the top of the optimal range for adult rainbow and brown trout. Other than these brief periods at these three stations, daily average water temperatures remained below stressful levels all summer at all six of the locations shown here.

Of course, water temperatures during the afternoon and early evening exceeded 67 degrees nearly every day for several weeks at Pinehaven and St. Anthony, but these daily warm periods were generally less than 6 hours in duration (roughly 3:00-9:00 p.m.) and followed by relatively cool temperatures in the late night and early morning hours. The highest instantaneous water temperatures recorded this summer were 73.4 degrees F at Pinehaven and 75.7 degrees F at St. Anthony. Both were recorded on the afternoon of August 2. These water temperatures were semi-stressful for the dominant trout species at these locations (rainbow trout and brown trout, respectively) but fell short of those that are stressful or lethal.

Mean daily water temperatures at all locations fell relatively gradually from early August until the end of the water year, although temperature at Island Park Dam dropped abruptly by around 5 degrees on September 8-9 and stayed there for most of the rest of the month. This abrupt drop was caused by a very strong cold front and accompanying wind event that rapidly cooled the surface of Island Park Reservoir, causing the reservoir to mix thoroughly and send the cold surface water to the bottom. This type of event is one of the few that will create large short-term changes in temperature downstream of large reservoirs.

Dissolved Oxygen

Dissolved oxygen followed expected seasonal patterns throughout the watershed, decreasing in response to warmer water temperatures as the spring and summer progressed (warmer water holds less oxygen) and increasing again in the fall. The pattern at Pinehaven is somewhat of an exception to this, as macrophyte photosynthesis, rather than temperature, dominates oxygen concentrations during the middle of the summer. Although not shown on the graphs of daily averages, dissolved oxygen there is very high in the afternoon when photosynthesis is at its peak but drops overnight, sometimes ranging from 6 mg/L in the morning to over 13 mg/L in the afternoon. However, even the graph of daily mean dissolved oxygen shows that it is relatively high and constant all summer at Pinehaven and does not show the systematic decline throughout the warm months as seen at Marysville, Ashton, and St. Anthony.

Dissolved oxygen at Island Park Dam is a complex function of reservoir dynamics, temperature, dam outflow point (gates vs. power plant), and aeration at the power plant. Large changes in dissolved oxygen at the dam reflect short-term and seasonal changes in these factors. The power plant operators are required to meet minimum dissolved oxygen standards whenever the power plant is operating and will use combinations of manual aeration and mixing of aerated water that spills through the gates to meet its requirements. Thus, dissolved oxygen at Island Park Dam stayed above 6 mg/L all summer, although just barely during times when reservoir oxygen concentrations were low and aeration was required to meet the standard. Elsewhere, daily average dissolved oxygen concentrations remained well above 6 mg/L, which us the bottom of the optimal zone for adult trout.

Phytoplankton Production

Phytoplankton consists of photosynthetic micro-organisims that include green algae, diatoms, and cyanobacteria, among others. Phytoplankton is important because it produces (the majority of Earth's) oxygen and forms the foundation of aquatic food webs. Our sondes record two measures of phytoplankton production: chlorophyll-a pigments contained inside viable phytoplankton cells and phycocyanin pigments contained inside of viable cyanobacteria cells. Cyanobacteria are sometimes called “blue-green algae,” even though they are not algae. Some cyanobacteria species can produce toxins, which at high concentrations can be harmful to humans and animals. Such harmful blooms can occur in Island Park Reservoir and Henrys Lake, as occurred during late summer of recent drought years, especially 2016. In 2020, we saw no evidence of harmful blooms. Taken together, chlorophyll-a and cyanobacteria indices provide a relative index of primary production in aquatic systems.

Patterns are highly seasonal, as reflected in the graphs. Productivity was highest in early May, coincident with warmer water temperatures, availability of sunlight, and availability of nutrients (primarily phosphorus and nitrogen) associated with spring runoff. High streamflow during runoff mobilizes nutrients contained in sediments on the stream bottom and also introduce nutrients from the land surface into streams. Secondary peaks in primary productivity occurred later in the summer, associated with localized events such as rainstorms (e.g., Pinehaven in late June) and reservoir dynamics (e.g., Island Park Dam in late September). Not surprisingly, the highest productivity in the river occurs at Island Park Dam and Pinehaven, where nutrients are most abundant because of concentration in and export from Island Park Reservoir. This high productivity is the reason why trout grow so fast and aquatic vegetation is so abundant in the river reach between Island Park Dam and Pinehaven, in comparison to other river reaches.


No water quality parameter is more important to anglers than turbidity. Technically, turbidity is just a measure of water clarity—the ability of light to penetrate the water. Higher turbidity means lower clarity. In the Henry’s Fork, turbidity reflects concentration of suspended material, both mineral sediment and organic material such as phytoplankton cells and decaying plant material. Although there is no specific turbidity value that is considered detrimental to trout, the state water quality standard for activities in the stream channel (e.g., bridge construction) is that the activity not increase turbidity more than 50 turbidity units above background. Our sondes measure turbidity in Formazin Nephalometric Units (FNU). The maximum turbidities we routinely record in our network are in the range of 20-25 FNU, usually during spring runoff and during freshet deliveries from the reservoir designed to mobilize and transport sediment out of the river. More typical values during the summer are 1-5 FNU, meaning that the state standard would allow turbidities as high as 50 FNU. Although very high turbidities (e.g., 100+ FNU) can limit light penetration into the water column to an extent that photosynthesis by aquatic vegetation is impeded, turbidities never get anywhere close to these values in the Henry’s Fork.

Concerns over high turbidity here are related to short-term effects on fishing quality and to long-term effects of mineral sediment, concentrations of which are correlated with turbidity. Anecdotally, the former occurs (or at least I hear about it) when turbidity exceeds around 6 FNU. The latter can be assessed only with analysis of sediment loads, river-reach sediment budgets, and long-term monitoring of aquatic invertebrate populations. As stated above, the latest information we have on this is presented in my blog from earlier this year. That indicates that sediment deposited downstream of Island Park Reservoir in 1992 is slowly being removed from the river, that annual springtime managed freshet deliveries from the dam can accelerate this process, and that the aquatic invertebrate community at Osborne Bridge has improved as a result. I will provide detailed analysis of sediment transport during 2020 in a separate blog.

At all sites except Island Park Dam and Ashton Dam, turbidity early in the spring and summer reflected streamflow. Turbidity spikes in April and May coincided with snowmelt, and downstream of Island Park Dam they also coincided with delivery of the managed freshet in early May. Spikes at Pinehaven and Marysville in late June were caused by strong thunderstorms on the afternoon June 21 that dropped heavy rain in the Island Park area. That rain event even increased turbidity at Island Park Dam, which had generally been at or below average up to that point. With the exception of turbidity associated with the September sediment event at Island Park Dam, turbidity downstream of Pinehaven was low and near average during July, August and September.

After the first of July, turbidity at Island Park Dam and Pinehaven was generally above the 2014-2019 average. Some of the spikes were associated with outflow increases from Island Park Dam during irrigation season and in particular with outflow increases that released more water through the gates. The largest of these occurred July 9-10, when outflow increased from 1,000 cfs to 1,500 cfs in two days. Phytoplankton blooms in the reservoir contributed to very small increases in turbidity later in the summer. Other than during the September sediment event, which will be the subject of a separate report, turbidity at Island Park Dam and Pinehaven was higher in 2020 than in any year since 2016 because Island Park outflow and reservoir draft during the 2020 irrigation season were by far the highest since 2016. July-August turbidity in 2020 was generally lower than that in 2016 and similar to that in 2014, in proportion to Island Park draft during those years. However, because outflow and draft have been much lower than average over the past three years, this year’s higher turbidities were perceptibly high to anglers, when compared with the past three years.

High turbidity downstream of Island Park Dam during September was unprecedented in our record and most likely the longest-duration, highest-magnitude turbidity event since the 1992 drawdown of Island Park Reservoir. Although coincident with reservoir mixing and associated export of organic matter, the September turbidity event was unquestionably caused by high export of suspended mineral sediment from the reservoir. The exported material was so fine it stayed in suspension throughout the length of the river (and even through Ashton Reservoir), as can be seen by high turbidity at Pinehaven, Marysville, Ashton Dam, and St. Anthony in mid- to late-September. Sediment from that event was still apparent at Ashton and St. Anthony on the first of October, three weeks after the start of the event.

My report on that event will include detailed analysis of possible causes, fate of that sediment as it moved down the river, and possible long-term consequences to the aquatic ecosystem. That report will also discuss ongoing and potential new management actions that could minimize the occurrence of such events in the future. Regardless of the details of this particular event, it was a reminder that all aspects of the fishery downstream are largely determined by management and limnology of Island Park Reservoir and that even greatly improved water management over the past few years—informed by increased scientific understanding—cannot prevent sediment stored on the reservoir bottom from being exported into the river downstream under certain conditions.

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