Last Fall, BVFF started a 5-year project to study gravel migration on the Boise and Owyhee rivers. Club member, Jeff Jones, lead an effort to drill 200 pieces of river gravel and epoxy PIT-tags inside them. Three-quarters of these rocks were placed in the Owyhee River last Fall and late this winter, a group of BVFF volunteers placed the rest of them into the Boise River, along with a cubic yard of spawning-size gravel. The resulting “study plot” was six feet wide by twelve feet long and four inches deep. Tagged rocks were placed in a grid at three different depths: On the surface, two inches below the surface and 4 inches below the surface. The study-plot grid below shows the layout of the study plot, quadrant, size of rock (small, medium, large) and depth of placement (eg: -4 inches).
Our plan is to work with Biomark and use a PIT-tag reader each Fall over the next five years to see where the tagged gravel ends up. The result will give us insights into how long our previous Boise River gravel additions stayed in the side channel and if the Owyhee river gravel is staying in place well enough to warrant doing a second augmentation.
Because of low snowpack, this Spring’s flows on the Boise river were lower than usual, but they were also of similar magnitude as 2021—which was the year we did our first Boise River gravel augmentation. Which means where we find the gravel this Fall is a reasonable estimate for where the gravel we added in 2021 ended up after the first year.
Summer flows on the Boise river typically run 800cfs – 1,000cfs, but because of the low snowpack last winter, water managers have already dropped flows on the Boise in town to 650cfs (which is a GREAT flow to fish—get out there!). At the end of June, I visited the side channel to take a look at the location where we added the cubic yard of gravel in a 6’ x 12’ rectangle. There was no visible rectangle of smaller gravels, but there were some smaller gravels on the inside bend where we placed gravel, but the further away from the bank you look, the larger the rock/cobble got (click on any photo to see a larger image).
Although I didn’t have a PIT-tag reader, I knew that in addition to the PIT tags, our rocks had a number written on them in permanent marker, to help us keep track of them while working without a PIT tag reader. I scuffed my wading shoes along the bottom to clear off a strip and picked up a few small rocks. Low and behold, I ran into one of our tagged rocks! B-44…. BINGO!
Comparing the number (B44) to the grid of tagged rocks we placed, it was a rock that was placed near the location I found it, but at the bottom of the 4 inches of gravel. Which means the higher flows this spring moved the gravel that was on top of it. (See the Boise River Study-Plot layout at the top of this article). It was tempting to dig through more of the rocks to see if I could find others, but that is best left until the Fall when we have the PIT Tag reader
Flows on the Owyhee won’t drop until mid-October at the end of Irrigation season. But I suspect that the gravel on the Owyhee hasn’t moved very much as we saw low spring flows this year. High spring flows occur less frequently on the Owyhee, so it will be a few years until we see what higher flows do.
My thanks to all of our BVFF volunteers that helped tag and place the gravel. It is a very valuable study for our club, and other organizations in the Treasure Valley are equally interested in learning the results! I am looking forward to our gravel “Easter Egg” hunt this Fall. I want to thank Nick Porter at Biomark for his help with this project. We wouldn’t be able to track down these tagged rocks without his help.
