I'm surprised that log jams can prevent fish migration however. Is this when they turn into waterfalls? Cheers, Paul
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Log Jams
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Paul Arden
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Log Jams
Super FP, Harps! I'm surprised that log jams can prevent fish migration however. Is this when they turn into waterfalls?
Cheers, Paul
I'm surprised that log jams can prevent fish migration however. Is this when they turn into waterfalls? Cheers, Paul
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Eagle Crest
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Log Jams
Paul,
Most fish don't have 'reverse'. If they enter into a 3-Dimensional maze of a log-jam, and they can't go forward and can't turn around, they die. It's the fish's propensity to seek slower flows to conserve energy, and in a log jam, that path is a result of restriction too small for them to fit through. The paths they can fit through, have the fastest flow, and they can't know if the flow path through the log jam is so far that it will cause them exhaustion. They typically won't attempt to pass in an upstream migration, even when we suspect it should be possible. Maybe little fish will ride the current going down-stream. I have an uncle that is a retired from the Nevada State Game Commission, and I can ask his opinion.
When I was lad knee high to a tall logger, I lived in Oregon logging country, and log jams in high gradient mountain streams were very common. The game commission used to dynamite big log jams on important spawning streams.
Fish ladders pose the same quandary to fish, and by comparison, they are hugely open compared to a log jam.
Most fish don't have 'reverse'. If they enter into a 3-Dimensional maze of a log-jam, and they can't go forward and can't turn around, they die. It's the fish's propensity to seek slower flows to conserve energy, and in a log jam, that path is a result of restriction too small for them to fit through. The paths they can fit through, have the fastest flow, and they can't know if the flow path through the log jam is so far that it will cause them exhaustion. They typically won't attempt to pass in an upstream migration, even when we suspect it should be possible. Maybe little fish will ride the current going down-stream. I have an uncle that is a retired from the Nevada State Game Commission, and I can ask his opinion.
When I was lad knee high to a tall logger, I lived in Oregon logging country, and log jams in high gradient mountain streams were very common. The game commission used to dynamite big log jams on important spawning streams.
Fish ladders pose the same quandary to fish, and by comparison, they are hugely open compared to a log jam.
Veni, Vidi, Pisci
Log Jams
Yeah, small log jams are okay, and I've seen trout back out of them... but it can get complicated.
Some of the bigger jams here are packed with large logs across the bottom and filled with smaller debris. These jams are so thick that only smaller fish can get through.
In a flood the bedload (gravel, sediment) packs the bottom of these big jams and water starts pouring over the top of the jam and creates a scour hole downstream. That can create a barrier (waterfall) in a small creek.
There is some great research in the Pacific Northwest that shows how the lack of large log jams (and all large woody debris) has changed how the rivers look and function. There is great images of old rivers and the logjams that create islands versus the new rivers without the tons of logs adding habitat diversity and stabilizing some of the beadload.
It's very interesting stuff!
Some of the bigger jams here are packed with large logs across the bottom and filled with smaller debris. These jams are so thick that only smaller fish can get through.
In a flood the bedload (gravel, sediment) packs the bottom of these big jams and water starts pouring over the top of the jam and creates a scour hole downstream. That can create a barrier (waterfall) in a small creek.
There is some great research in the Pacific Northwest that shows how the lack of large log jams (and all large woody debris) has changed how the rivers look and function. There is great images of old rivers and the logjams that create islands versus the new rivers without the tons of logs adding habitat diversity and stabilizing some of the beadload.
It's very interesting stuff!
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Paul Arden
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Log Jams
Rivers changing course must be an important part of how they have functioned since time began. In an ideal world would the best management be no management?
Cheers, Paul
Cheers, Paul
Log Jams
No Management is always the best!
But...
Unfortunately, in places with dams there is less bed materials drifting downstream, so less "mobile fill" to stop scour and create riffles, etc.
Where an area has been logged, the flows peak faster, causing higher- more erosive- velocities that may have to be managed.
There are also less trees/woody debris floating down, that should be creating habitat along the banks and in the river channels.
With those to issues, rivers start to "entrench". As they dig deeper, there is less ability to access the floodplains, less ability to change course, and the more contained flows cause high velocities and more scour and trenching. That leads to less wood in the river, less gravels, faster flows, etc, etc. Eventually there is downcutting or a bedrock bottom, that limits what species can use the watercourse.
So... short answer. Where we've screwed things up, we sometimes have to help to fix it... whether encourage beaver dams, build logjams, construct grade control riffles, or add gravel back into the stream.
But...
Unfortunately, in places with dams there is less bed materials drifting downstream, so less "mobile fill" to stop scour and create riffles, etc.
Where an area has been logged, the flows peak faster, causing higher- more erosive- velocities that may have to be managed.
There are also less trees/woody debris floating down, that should be creating habitat along the banks and in the river channels.
With those to issues, rivers start to "entrench". As they dig deeper, there is less ability to access the floodplains, less ability to change course, and the more contained flows cause high velocities and more scour and trenching. That leads to less wood in the river, less gravels, faster flows, etc, etc. Eventually there is downcutting or a bedrock bottom, that limits what species can use the watercourse.
So... short answer. Where we've screwed things up, we sometimes have to help to fix it... whether encourage beaver dams, build logjams, construct grade control riffles, or add gravel back into the stream.