The 140,000 acre Riverside Fire will potentially have a variety of effects on fish in the Clackamas River.
During an online event with the Oregon Wildlife Foundation Thursday, Oct. 29, Portland General Electric Fish Biologist Garth Wyatt and Mt. Hood National Forest Fisheries Program Manager Bruce Zoellick discussed the impact of the wildfire that began in early September and is currently 72% contained.
Wyatt described the wildfire as having "a mosaic of effects" on the coho, salmon, steelhead and other fish that call the Clackamas River home.
"We're seen some basins get hit really hard, and others are unscathed," Wyatt said. "(The impacts) are hard to say yet. We don't want to paint too bad a picture, but no doubt about it, there's going to be a lot of change here in the next 6-8 months in the upper basin."
Zoellick emphasized the unprecedented nature of the fire, and described it as potentially being "a basin resetting event."
"It will be interesting to follow the effects over time," he said.
Because some areas burned more severely than others, there will be varying levels of impacts for different areas of the Clackamas River. Areas that burned more heavily may see a loss of overhead vegetation that provides beneficial shade for fish.
"In some of the smaller tributary streams, if it burned super hot there potentially was some mortality of fish themselves, but it's likely that most fish didn't experience temperatures that would cause mortality. It's more the short term elimination of food and complexity and cover from streamside vegetation," Zoellick said.
Potential impacts from the wildfires will also depend on weather conditions this winter.
"You can often have landslides that occur, which may actually disconnect some of our smaller tributaries for a certain time period," Wyatt said. "Those landslides can result in really heavy dirt coming off the hillside and actually setting on redds, which are where fish stick their nest and lay their eggs. It reduces the amount of water flow and oxygen through there, which then can impact the survival of that particular spawning pair ... Hopefully we don't see extremely large rain on snow events where you would probably get a condition where you would be fairly prone to landslides."
Zoellick added, "the next three winters are going to be really interesting."
"Some of the sub basins burned with an intensity that 75% of the base and all the understory vegetation and organic matter on the soil burned, so you're just left with mineral soil," he said. "So potentially if we have a heavy rain storm, water will be delivered much faster to the channels. You'll have much higher peak flows."
Zoellick noted that one location in the Clackamas River area he's concerned about is Fish Creek, an area frequently used by the river's steelhead.
"It's historically been a big producer of winter steelhead, but it has relatively unique geology," he said. "It has a lot of areas prone to landslides, and it was one of the sub basins that burned the most severely. 75% of that basin had severe burn intensity, where all the organic matter and ground cover was burned up, so you're just left with mineral soil."
Zoellick added that Forest Service officials may place trees that died in the fire in the river's channels to hold some sediment in place, and Wyatt said that trees near the Rainbow and Ripplebrook campgrounds that have tipped into the river may create beneficial habitat complexity for fish.
"When you have wood across the channel, it makes it a lot harder for the river to come off the mountain as easily as it wants to and so the gravel that it's carrying during high flows settles out in the channel upstream of the wood jam, and then all of a sudden you've got travels the salmon and steelhead use to spawn," Zoellick explained.
Wyatt said that one fortunate aspect of the fire was that most of the Clackamas River's spring chinooks and coho had already moved through much of the impacted areas.
"I think we're somewhat lucky that the majority of our spring Chinook adults are doing so upstream of the distribution of the fire," he continued. "Roughly 70% of our Chinook are spawning from the Oak Grove fork, which was largely spared ... Our early run coho, for the most part, have already slipped upstream too. They predominantly spawn in the upper part of the basin."
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