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High Turbidity and Sediment Event During Memorial Day Weekend

The main message: Spring of 2018 brought the highest runoff event in 7 years to the upper Henry’s Fork watershed! Our network of water quality monitors showed that these flows were strong enough to provide a major springtime sediment flush--a natural rhythm of our local hydrology that provides significant benefit to trout and aquatic insect habitat. These favorably high natural flows came in two periods during April and May of this year. Unfortunately, the second period coincided with Memorial Day weekend—a reality that put a murky, cloudy, and disappointing damper on dry-fly fishing.

So, why did this high flow and high turbidity event coincide with the Memorial Day weekend?

Island Park (IP) Reservoir was already full when the watershed received over an inch of rain on May 23–24. The resulting increase in inflow was necessarily passed through IP Reservoir. For better or worse, IP Reservoir was not designed to discharge over the dam. Thus managers lack even the operational and mechanical option to pass the necessary flow over the dam. (Also this would flood the road over the top of the dam.) Bottom release does deliver more turbid water, but also much cooler water than from surface release. As many of you already know, the downstream fishery benefits year-round from this temperature moderation provided by IP dam’s bottom release.

The details

If you’ve been reading Rob’s daily water reports, you know that natural inflow to the upper Henry’s Fork (mostly snowmelt) began to sharply increase in mid-April reaching what we thought would be the season’s peak in early May. Then on May 23rd we experienced a widespread large rain event that lasted through May 24th. This event brought an average of 1.2 inches of rain to the watershed (total 3-day precipitation averaged across all 12 SnoTel sites) and the Ashton gauge recorded whopping 1.7 inches! Both large natural flow events I mention can be seen in Figure 1.

Figure 1. Daily average flow (cfs) by reach.

Our network of water quality monitors showed that these flows were strong enough to provide a major springtime sediment flush, a significant benefit to trout and aquatic insect habitat. A spring flushing flow is part of the natural rhythm of the Henry’s Fork. However, these high spring flows are usually captured to fill Island Park (IP) Reservoir ahead of irrigation demand, resulting in only moderate downstream increases. This year, IP Reservoir filled quickly as a result of high winter carryover and natural inflow, so that these particular high flows were largely passed through the reservoir.

These spring flush events were evident through observed increased turbidity, suspended sediment concentrations, and suspended sediment loads. Recall that we collect weekly field samples of turbidity and suspended sediment concentration (SSC) and our sonde network records turbidity every 15 minutes. Using the weekly field sample data, we are able to develop a statistical relationship between turbidity and SSC and then use the daily sonde turbidity data to estimate daily SSC. Recall that turbidity is a measure of how far visual light penetrates through a liquid. Therefore, we use this visual index to estimate a concentration of actual material in the water column. SSC from 7 reaches in our sonde network are shown in Figure 2. We then combine daily flow data (from USGS and Fall River Electric) with our estimated daily SSC to derive estimated suspended sediment loads—tons of sediment moving through a given reach per day. Suspended sediment load by reach is shown in Figures 3 and 4. You’ll notice we weren’t able to derive sediment loads from our Warm River or Flat Rock sonde data since we lack flow data in those reaches.

Over the period from April 1 to May 28, sediment concentrations were consistently lowest below both IP and Ashton reservoirs and in Buffalo River (Fig 2). Highest concentrations were observed in late April at our Pinehaven and Marysville sondes following the first flushing flow. Flat Rock displayed high concentrations during this period and a little afterward due to high snowmelt and again after flow increased out of Henry’s Lake outlet, in response to high natural inflow. The next period of highest concentrations were after the late-May rain event when we again saw increases at Flat Rock, Pinehaven, and Marysville. Notice that by this late May flush, a large amount of sediment had already been mobilized and flushed from Harriman to Pinehaven since concentrations weren’t nearly as high as they were in late April (Figure 2, orange line) yet flow was just as high (Fig 1, orange line).

Figure 2. Daily average suspended sediment concentrations by reach.

So what happened?

Why were sediment concentrations higher at Pinehaven and Marysville than below IP reservoir? Growth of macrophytes (rooted aquatic plants) is heaviest from Harriman Ranch through Pinehaven and these plants displace water, raising the depth of the reach and slowing down water velocity. Thus material has more opportunity to settle out of the water through these reaches. This material is also held in place by the plant roots, and as the plants grow, they further slow velocity, allowing material to accrue all year. Most of this plant biomass dies over the winter, so when the first high flows of spring wash through, the accrued material gets mobilized and flushed. Again, these high flow events were the highest upstream of Ashton Dam in 7 years, so they were flushing several years worth of accrued material.

Sediment load, tons per day

There was an increase in sediment load out of IP reservoir (Fig 3, bottom magenta polygon) coincident with increased discharge beginning May 24th. However, this increase was dwarfed by the increase in load picked up between Buffalo confluence and Pinehaven. Therefore if you were fishing, say, Pinehaven or Warm River to Ashton, most of the sediment you noticed had been flushed out of Harriman Ranch and downstream reaches since sediment discharged out of IP reservoir was less than half of the total during late May. So, yes, downstream reaches were more turbid in response to the increased discharge out of IP reservoir. However, the timing of this increased discharge couldn’t be helped since the reservoir was already full when the watershed received over an inch of rain on May 23–24. Additionally, passing inflow over the dam is not an option at IP dam, so both the high flows and high turbidity during Memorial Day weekend were ill timed and impossible to avoid.

Figure 3. Daily average total suspended sediment load by reach. The polygons are additive, so the very top is the total load moving through the river upstream of Ashton Reservoir (itself) and each polygon represents contribution from that reach.

It is worth mentioning that USBR could have ordered a small decrease in outflow on Monday in response to a small decrease in inflow as the effect of the rain wore off. HFF did ask for this decrease in outflow, but USBR makes decisions based on several factors since IP reservoir is a single node in the regional network of reservoirs they managers.

What happened?

The May 23rd increase in discharge was in response to the sharp increase in natural inflow caused by the widespread rain event and served as a second flushing flow through the Harriman Ranch and Pinehaven reaches, actually moving sediment out of those reaches. As already said, this is a highly beneficial natural annual pattern that clears fine sediment out of gravel improving aquatic insects and trout habitat. Unfortunately, this natural event (over an inch of rain in 24 hours) was very badly timed for the fishing community by mobilizing sediment in the steam channel resulting in very cloudy and turbid water during one of the most popular fishing weekends of the year.

It is important to remind you that increased turbidity and sediment such as this do not have negative biological impacts on Henry’s Fork fish individuals or on their long-term population trends. The late April and late May sediment events were indeed two of the highest we’ve seen on the upper river in several years, but were still low on the scale of concentrations that juvenile and adult trout can handle in the water column. Biological impacts associated with high turbidity/sediment that HF trout may experience are the result of build up of fines that can reduce spawning-to-emergence survival and reduce populations of aquatic insects by reducing quality of their gravel habitat. Rather, this flushing event will have a positive impact on population trends by moving detrimental fines out of the stream channels.

The other interesting point is that Ashton reservoir—a run-of-the-river reservoir that was designed to pass inflow over the top of the dam—stored a huge amount of sediment. The highest sediment loads in the Henry’s Fork passed by our Marysville sonde (Fig 4, blue polygon and Fig 5, blue line) and approximately half of that sediment was stored in Ashton reservoir (Fig 4, yellow polygon). We did hear from anglers (including our in-house anglers) how turbid the water was through the Ora reach (and our Ashton Dam sonde data clearly confirmed this), but clarity would have been much worse without Ashton reservoir trapping half of the mobilized sediment!

Figure 4. Suspended sediment mass trapped in Ashton Reservoir as the difference between load at Marysville and load below Ashton Dam.

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