Main message: The great spring sediment flush we observed earlier this year not only impacted fishing during Memorial Day weekend—by contributing to high flows and turbidity—but has continued to impact the fishing experience from Box Canyon through Riverside this season in two plainly obvious ways: depressed hatches and higher than average amounts of uprooted floating plant material. In addition, the rainy and cloudy weather that contributed to the spring flush may also have contributed to later hatch timing of some species earlier this summer.

Recall my blog from June 8 where we quantified the amount of sediment that was flushed out of each reach of the Henry’s Fork, from Island Park Dam to Ashton Dam, during the major runoff events we experienced during April and May. These spring rains brought the highest spring runoff flows 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. We’ve seen much higher summertime flows than these in the last 7 years, but what was significant about this spring’s flows is that they were the highest we’ve seen in April and May—before macrophytes (rooted aquatic plants) had grown and stabilized the substrate. Watershed-wide, the largest amount of sediment was flushed from the stream channel between Buffalo River confluence and Ashton Reservoir. I have reproduced below Figure 3 from my June blog (renamed Figure 1 for this blog), which clearly displays the in-stream sources of the flushed sediment.

Figure 1 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.


For the remainder of this blog I’ll focus on the river upstream of Mesa Falls, since this is where we have witnessed later and depressed hatches and high amounts of uprooted floating plants.

We calculated that a total of 1,900 tons of sediment were flushed out of the stream bottom from Box Canyon to Riverside between April 1 and May 28 of this year! Then, in my blog from July 16 I explained that we observed high amounts of floating plants from roughly mid-Box Canyon through Riverside. In an unrelated study this summer, we compared percent cover of macrophytes in the river at Last Chance on July 13th to July 20th. We observed that plant cover actually decreased at Last Chance during this period. Typically this time of year is marked by heavy macrophyte growth, not decay. Using information from the sediment budget calculations, the percent cover study, as well as anecdotal evidence from anglers and members, we put two and two together and realized that the springtime sediment flush removed fine substrate that these rooted aquatic plants have used to anchor themselves to the bottom in previous years. Recall that as macrophytes grow during the summer, they displace water, which raises the stage of the river and slows velocity. This allows fine sediment to settle out of the water column into the stream channel where it is stored until the following spring run-off (before plants have grown back) flushes some of it out. We have observed this cycle in our sonde data for years. Then the flows this spring were so high that they cleaned out much more sediment than the comparatively lower spring flows of the last 7 years. Spring flows this year loosened up the substrate by removing the finest sediment, so plants had less consolidated substrate to root into. The plants continued to grow through May and June, but at some point they grew too large to remain anchored in the looser substrate against the steadily rising summertime outflow from Island Park reservoir. We first heard reports of large amounts of plant material—individual pieces as well as mats or rafts of plant material—around July 10th, the day total discharge was increased to over 1,000 cfs. For better or worse, 1,000 cfs was the threshold flow for these plants: below this they could hold on, above this many became uprooted. During the following couple of weeks, flows continued to increase, reaching their peak between July 17th and the 22nd, but macrophytes continued to grow. This combination resulted in the continual uprooting of more and more plants from the stream channel that we have been experiencing.

From our own fishing experience as well as reports from anglers and members, the impact on fishing of this floating plant material has ranged from minor to very aggravating. Perhaps the larger issue this year was the later timing and depressed amount of several of the familiar summertime hatches at Last Chance, through the Ranch, and at Pinehaven, compared to the previous 4–5 years. Using our sonde temperature data, we observed that May and June temperatures from Box Canyon to Pinehaven were cooler this year compared to the last three years (limit of our sonde records) due to the cloudy and rainy weather we experienced during those months. Based on literature, as well as our own research on the Henry’s Fork, it is understood that cooler water temperatures delay hatch timing of adult aquatic insects. The mechanism is known as accumulation of degree-days, where time spent in water temperatures above a species-specific value drive the metabolic changes required to “hatch”.  We think this could explain the later PMD and Green Drake hatch that has typically occurred between the Ranch opener and July 4th.

Also, we heard reports that the PMD hatch was not only late this year, but was significantly depressed in numbers of individuals. We think the spring sediment flush could have impacted the size of the PMD hatch this year. If we observed 1,900 tons of sediment get flushed out of the stream channel from Box Canyon to Pinehaven this spring, it is possible that several types of nymphs were also suddenly displaced, which would result in lower numbers of certain species. Finally, less substrate during the growing season may have also resulted in fewer macrophytes overall, which could affect some insect species (certain mayflies and caddisflies) that use the plants as habitat.

It is important to remember that the benefits to insect and trout habitat we predict will result from the great springtime flush would not occur as early as this year. For instance we expect to observe next year that the cleaner gravel has resulted in higher relative numbers of fine-intolerant species of mayflies, stoneflies, and caddisflies and lower amounts of midges, worms, and Brown Drakes, which favor fine substrate, from Last Chance to Riverside. Each spring, we conduct benthic macroinvertebrate sampling at 5 locations along the Henry’s Fork, including Last Chance and Osborne Bridge.  This sampling allows us to estimate the amount and relative abundance of macroinvertebrate nymphs each year before they hatch. Due to the cleaner substrate, we expect we will see an increase in relative abundance of PMD, Green Drake, and Flav nymphs at the Osborne Bridge location. Hopefully this will also result in better hatches of these species, improving the fishing experience.

We also expect cleaner gravel to benefit trout spawning success, but we won’t be able to detect this for a couple of years to come, while we continue to work to achieve optimum wintertime flows to help these fingerlings survive to become fishable age/size trout. The springtime flush was a large alteration to a natural system, and thus will have several short-term and longer-term impacts, some easy to predict and others surprising. However, we expect the longer-term impacts to be beneficial for insect and trout populations. These expectations are based on well-established interactions; we have good reason to expect these boons to the ecosystem (substrate with fewer fines) will also translate into some benefits to the fishing experience (more mayflies, stoneflies, and caddisflies, more fish to cast to).