South Fork of the Boise Blog, Vol. 3
South Fork of the Boise Seining Session - Wednesday, January 29th, 2014

Over the last two months I have been researching and reporting about the health of the South Fork of the Boise River after fires and mudslides this past summer and fall. We have established that there was not a massive fish kill that has made the fishing poor. I have actually found that the fish in the system are fat and healthy.

One question that repeatedly came up was, “What are the effects of the fires and mudslides on the insect life in the river system?”. So, with the help of a few friends, I was put in touch with Matthew Laramie. Matt is a stream ecologist who specializes in Aquatic Macro-invertebrate Taxonomy. What that said to me was he's pretty smart and he might be able to shed some light on what is going on in the gravel out there on the South Fork. We picked a day that worked for both of us, and set out to examine the river. Unfortunately, it was a day filled with freezing fog, sleet, rain, and icy roads. Despite the weather, we took a ride out to the South Fork of the Boise to see what we could find.

Let me start by saying that this outing was just that, an outing. This was not a scientifically conducted sampling of the insect life. A properly conducted scientific sampling would need to be started with a baseline sampling before the fires and mudslides and would last into the future for several years. What I was hopping to get out of this trip to the South Fork was some basic insights into the effects of the fires and the mudslides on the insect life in the river. We felt the best way to get some idea about what is going on would be to sample areas with different levels of disturbance. We chose three areas to sample that would be recognizable to people interested in this topic. We decided on an area near Danskin Bridge, an area near Indian Rock, and an area near Reclamation Village. We used a standard river insect sampling seine at all three places.

The river at Danskin was “colored up” from the tributaries that were pumping mud into the river caused by the runoff from the rain that day. The Danskin Bridge area is also directly down stream from some of the mudslides that were responsible for massive amounts of materials entering the river. We walked down to the first riffle below Danskin Bridge for our first sampling. After disturbing the rocks in the riffle, we went to the bank to see what we had captured in our seine. Using a shallow white pan, we emptied the contents of the seine. We found numerous bug species. There was massive amounts of organic matter or “detritus” in the seine. All the debris made it hard to see many of the small insects in our sample, so it was difficult to get a precise reading. Despite all the organic debris we did find a variety of stoneflies, mayflies, caddis, chironomids, and aquatic worms. The pan wasn't loaded with insects, but the bio-diversity in the pan was good. Seeing bugs at all was good news. The slides above Danskin Bridge were massive and “paved” over the river bottom burying many life forms that made the cobble their home. The presence of insects at all is a testament to the resilience of the stream and its ecosystem.

The next sampling spot was Indian Rock. The reason we chose this spot was because the mudslides and fires had impacted this portion of the river very lightly. We walked just up from the boat launch in the fast water. As we expected, this spot was full of insect life. We found a Sculpin, bunches of stoneflies, caddis flies, chironomids, and aquatic worms. The seine was mostly free of silt, sand and other organic material. The seine was almost all insects. This spot had by far the most insect life.

The third spot was the slide area upstream of Reclamation Village. We went to the downstream side of the mud slide where the debris entered the river. We repeated the process of moving into riffled water, placed the seine into the water, and then began the sampling process. In this area, the water column was clearer than it was at Danskin Bridge, but the cobble on the river bottom was heavily clogged with small gravel and silt. However, we still found bugs in the seine. It seemed like more bugs than at Danskin, but less, by a lot, than the sample at Indian Rock. I was very surprised at the health of the river at the place where one of the major mudslides entered the river. I caught a nice 19” Bull Trout a few weeks earlier within about 30 yards of where we seined.

So, what can we infer from what we found at these three sampling spots? Again, this was not a scientific experiment. This was just two concerned fishermen going out to look at some bugs. Matt and I were both happy to find that all three samples had decent numbers of insects in them. Matt pointed out that, although the three samples did not have the same biomass, the biodiversity was in the samples was good. Good biodiversity in a stream is usually an indicator for overall stream health. Matt also pointed out that the stoneflies, which live for several years in the gravel before hatching, all had different year classes represented in the samples. How could this be that there are bugs in some of the most affected areas? Aquatic insects periodically let go of their rocks and drift downstream. This occurrence is referred to as "periodic drift". There are many factors that influence periodic drift. These factors include carrying capacity, population density, time of day, and catastrophic events such as water fluctuation, pollution, and other large events such as fire and mudslides. This may explain why, even in areas where the mud slide material completely paved over the old cobble, the river bottom still had a good diversity of insects.

Matt said that, overall, the river looks to be in good shape. He has previously worked at river sites that have been completely scoured, down to bedrock, and seen them recover in a timely fashion. Matt and I both agree that the South Fork of the Boise River, although affected by the fires and mudslides, has not been devastated. We continually noted that the disturbance did not seem to be river wide. In fact, the first half mile, or so, of the river is still as it was last year at this time, minus some burnt trees. This area should have all the insect stocks to replenish any part of the bug population effected by the mudslides. One question that Matt did raise was, “Once the remaining insects mate, will their eggs find good clean cobble to propagate in this year?”. The amount of clean cobble will depend primarily on the amount of water released out of the dam this spring. Good flows would allow the silt to be carried downstream into Arrowrock Reservoir and onto the banks of the river. Matt was concerned about what will happen if we do not have enough water to allow for good flows this spring. When the insects lay there eggs, will the eggs have anywhere with good, oxygenated water flow, so the eggs can grow and hatch? There will definitely be areas for insect eggs to come to rest in ideal conditions, but the question remains, how much of the real-estate is going to be ideal for insect propagation? Higher water levels this spring would clean out the gravel for spawning trout and insect propagation.

This leads us to the next question, “Will there be enough water in Anderson Ranch Reservoir this spring to allow a flush?” Anderson Ranch Reservoir on Feb 1, 2014 was at 24%, or 95,813 acre/ft. Last year on Feb 1, 2013 Anderson Ranch Reservoir was at 233,237 acre/ft. Let's hope the forecast for the next few months is a precipitous one, so we can at least run a decent amount of water through the dam this spring. Having good spring flows will start the process of purging the system of cobble clogging silt and debris. At this time, it does not look likely that there will be enough water stored in Anderson Ranch Reservoir to allow for a good spring runoff. I plan to speak to the Bureau of Reclamation about their plan for this spring and summer in my next installment of the South Fork of the Boise Blog.

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