Effects of fire on a stream

[NohackleHS 0]

I live between San Francisco and Sacramento. The stream closest to my house, Putah Creek, is in the hill country not too far from Napa and is currently engulfed in flames. One fellow who was able to see some of the damage said that much of the fire had burned right down to the waters edge. At other locations the fire was hit or miss. Of course, many local anglers have asked what effect the ash will be when the Winter rains deposit it in the stream. Does anyone have any thoughts about what effect this fire might have on the stream?

[FlaFly 0]

I can only speculate, but I'm sure given the frequency of wild fires in California that the local stream biologists have studied that issue.

[FlaFly 0]

I googled the issue and found number of scientific reports. The primary issue would appear to be increased runoff of sediment to streams, due to destruction of plant cover. Excessive sediment would essentially destroy benthic insect larvae, reducing food for fish.

[mikechell 0]

There will be a big fish kill after the first few rains. With the current drought going on, and the lack of grass growth already, it's going to be devastating to the creeks in the area.

[FlaFly 0]

Streams experience a phenomenon called stream drift. If fauna are killed off in a given reach of stream, they will over time be replaced by insects, larvae, etc., drifting down from undamaged reaches upstream and from tributaries. Unfortunately if the hard substrate, rocks, snags, etc., have been covered, only those species able to live in soft sediment will survive. Since it requires more energy to bring a particle of sediment into suspension than it does to keep it there, the soft sediment is likely to hang around for a long time... until there is a big enough storm to increase stream velocity sufficiently. Then of course all the sediment will move downstream into a lake (reservoir) or estuary, where it will hang around MOL permanently.

I'm assuming California is like Florida in that years of fire suppression has resulted in accumulation of fuel that results in major wildfires. Nature, via lightning, has a way of occasionally starting fires, which tend to be small if fire is not suppressed. Here in Fla the state/counties conduct "controlled burns" to simulate nature in a fairly safe manner, and to keep the fuel from accumulating.

[Bruce Norikane 0]

Depends on topography and extent of fire in the watershed upstream.

 

One of my local creeks suffered a bad fire a number of years ago that burned the slopes above right down to the banks. The stream took a direct hit from a slurry bomber resulting in an immediate fishkill. Unfortunately, big fish were the hardest hit and the photos of dead 16"+ trout along the banks far downstream was truly sad.

 

The good news is despite an extensive burn, this creek was never hit by disastrous after effects of extreme sedimentation or heavy ash deposits. The bugs recovered quickly and the small trout as well. The area was closed the next year while the trails were cleared of charred trees, but fishing was good when it re-opened. The really big fish have never re-appeared, but the bug life is fantastic, so it's a healthy mountain creek.

 

On the other hand, in 2002, the gigantic Hayman fire devastated the S. Platte River in Colorado. It was the biggest fire in Colorado history, and it burned much of the watershed. For years after the fire, huge sedimentation loads washed into the river and reservoirs choking the riverbed in many areas. Some reservoirs were filled with ash and charred wood.

 

Some parts of the river recovered fairly quickly, and some took many years. The famous and popular Deckers area took forever to recover, but it's been fishing great the last few years. http://www.denverpost.com/willoughby/ci_20504813/south-platte-river-fishing-since-hayman-fire

 

I wish you the best of luck and hope your stream is closer to the former with little long term damage, but only time will tell. BTW - watch out for flooding. When the trees and grass are gone, even a little rain can accelerate into a flash flood.

[wschmitt3 0]

Mike is probably right. If the burn was really destructive there will likely be lots of sediment and ash washed into the river and with the next rain it will probably raise temperatures, reduce oxygen and make the water too alkaline (if memory serves) and it will possibly kill quite a few fish but nature has a way of coming back so it will probably bounce back in a few years.

 

There was a really good story about fires and streams in the Summer 2015 Fly Rod and Reel titled "The Survivor".

[FlaFly 0]

schmitty.... true if all other things are equal. Other variables that could affect how the stream fares are: how big an area was burned, the slope of the burned area, amount of rainfall on the burned area, velocity in the stream itself... probably others.

[wschmitt3 0]

Very true flafly,

[SilverCreek 0]

Effects depend on

 

1. The gradient of the stream

 

2. The amount of top soil and debris that can wash into the stream

 

3. Whether the fire left any protective corridor of intact vegetation next to the stream adn teh width of that corridor.

 

4. Whether the following rains dump enough water to create runoff into stream.

 

The steeper the gradient, the thicker the soil and debris, the less intact the protective corridor, the greater the subsequent rains ----> the greater the amount of debris that will wash into the stream. It is not only dirt that washes into the stream but vegetation, sticks, logs, etc that can detroy the and cover up the gravel spawing beds and smother the aquatic insects. Loss of forest cover leads to warming of the water.

 

The greatesrt natural disaster is not from forest fires but from volcanic eruptions such as Mount St Helens (1980) that destroyed river and forest life, leaving a mountain of ash and mud flows that takes decades to recover from.

 

In the pyroclastic flow and debris avalanche zones, most of the streams had chronic high levels of fine sediment and shifting channels. By 2005, these streams had not developed conditions suitable for fish, and decades will likely pass before these streams can support fish.

 

http://www.fs.fed.us/pnw/mtsthelens/faq/q10.shtml

[FlaFly 0]

... all of the above, plus the pH will probably be changed dramatically. Silver.... pH higher due to potash? of lower? What do you think?

[SilverCreek 0]

Ph I think will depend on the minerals in the ash which varies with what type of rock is erupted and on the gases accompanying the eruption which are mainly acidic. So no way to say what the PH will do.

[wschmitt3 0]

Not volcanic ash silver, from forest fires, I believe that wood ash will make the water alkaline.

[FlaFly 0]

that's the way Appalachian old-timers used to make lye... potash in water. I was iffy though because all that carbon-based stuff in the ash might tend to be acidic. Maybe some of the scientific papers on wildfires might have some info.

[NohackleHS 0]

Thanks for the replies. There's an online forum for Putah Creek and one of the guys on the forum posted pictures of the stream. It appears that most of the burnt areas are in the hills surrounding the stream. Some of the pictures showed some burnt out paths along the stream. In all honesty, the fire damage didn't appear to be as bad as I was envisioning. However, the west side of the stream was the most severely affected and I don't think the pictures included that side of the stream. In any case, I guess only time will tell what the effects of the fire will be. The creek is only an hour from my house and for a stream so close to the bay area and to Sacramento, it has a surprising number of large rainbows. Thanks again for the replies.