This fall HFF initiated a new water-quality monitoring study designed to answer the following question: How far downstream of Island Park Dam (IP Dam) do high turbidity levels persist? I introduced this study in my blog post made on September 9, 2016. Please find details of the background and motivation for this study, as well as a map of the sampling sites, in the previous blog post. The current blog post summarizes what we found during our 2016 data collection season.
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.
HFF is two years into a long-term program of monitoring aquatic macroinvertebrates at Flatrock, Last Chance, Osborne Bridge, Ashton, and St. Anthony.
Primary conclusions from comparison of 2015 and 2016 data are:
Abundance of macroinvertebrates averages about 47,000 individuals per square meter of stream bottom across all sites.
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.
Aquatic habitat quality ranges from excellent at Flatrock to good at St. Anthony, decreasing with distance downstream from the headwaters.
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.
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.
The aspens are yellow, the mornings are frosty, and the Brown trout (Salmo trutta) are staging to spawn. Brown trout, like many of their salmonid cousins, migrate from lakes, oceans, or main-stem river reaches to headwater habitat to dig redds (nests in the gravel) and deposit their eggs1. For Brown trout in the Upper Snake River region, many don’t have access to headwater habitat due to hydraulic barriers and will spawn in main-stem river reaches instead.