What effects do floods have on insects?

[randyflycaster]

I'm reading Leonard Wright's The Dry Fly as a Living Insect. In his book he briefly mentions that when rivers get flooded much of the insect population gets washed out.

I wasn't aware of this. As it turns out, a river I often fish was heavily flooded in April. It seems to me that I've seen a lot less insect - and trout - activity this season. (Fishing has been horrible.) I'm now wondering if it was the flood that did the damage. If so, how does the insect population recover?

Thanks,

Randy

[lastchance]

I'd say, if that were true, we wouldn't have many places left to fish. I'm no entomologist, but I'd have to disagree with that. Just my opinion.

[xfishcop]

It seems to me that a heavy muddy water flow can decrease the population of insect life in the river. Around here the caddis population is very diminished. We have just had a series of flash flooding which has resulted in muddy flows. I turned over a few rocks and was mainly seeing clinger mayflies. It would appear that the mud may smother the bugs????????
Dick

[johnstoeckel]

I generally agree with Wright, but it also depends.

Flooding can wash away insects, particularly when they are in an active, vulnerable stage. When you see stream sections where the rocks have been scoured clean, you can bet that many of the clinger mayflies, cased caddis, etc. have been scoured away as well. These insects will be down in numbers for a while, but the flood doesn't get them all so they bounce back. In fact, flooding may clear out silt and create more insect habitat in the longer run. I think a lot of this depends on when the flooding occurs relative to the insect life cycle.

My impression is burrowing type flies -- some mayflies, crane flies, midges -- are generally less effected by flooding.

Also depending on where you're at and the time of year, flooding can wash high levels of farm chemicals or manure into the streams which can be toxic for insects as well.

[zaxaca]

I read an article in the past couple of years (tried unsuccessfully to Google it) about how changes in tailwater releases on the Arkansas River in Colorado had a tremendous negative impact on the infamous Mothers' Day caddis hatch. My recollection is that it was thought some early water releases (very cold water) had altered the hatch cycles of the caddis reducing the quality of the hatch. So, it was posited that the timing of the water temperature changes, rather then the flooding/rising water, had reduced the hatches. This made sense to me since the river deals with the spring runoff annually. Having fished the Mothers' Day hatch for years, the quality of the hatches has been reduced considerably since about 2006 or so. I've heard from others that it might be improving.

[WarrenP]

As I see it, in my head: The insect life starts in the sediment at the bottom of a river which could be mud, gravel or sand. When the river rises slowly, the moving water does not affect the sediment that much, but, if the river rises quickly, as in a flash flood, it will wash away a lot (not all) of the bottom sediment and move this sediment and insects living in it, further down the river. The insect life will come back as soon as the river recedes to it's normal level. Now a tailwater system, below a generating dam, will be affected the same way plus the changes in water temperature, when generating, just adds to the problem. Just my thoughts and nothing more....

[Silver Creek]

Aquatic insects washed downstream by flooding falls into the category of CATASTROPHIC DRIFT. The opposite of flooding is drought and it can also cause catastrophic drift, as can pollution. CATASTROPHIC DRIFT is one form of insect drift with the other two being Constant Drift and Behavioral Drift. Behavioral and Constant Drift allows the insects to colonize the entire river.

Downstream drift is why insects fly upstream to mate and lay their eggs. This upstream egg laying behavior both corrects for downstream drift and distributes the insect eggs so they eventually inhabit the entire stream. Downstream drift and upstream egg laying are adaptive behaviors and the upstream egg laying re-established the insects upstream after flooding.

A fly fisher can take advantage of behavior drift to increase the chances of nymphing success. Behavior drift peaks after dusk and before dawn, at dark so the insects are less likely to be seen. Again this is an adaptation that insures that the fewest drifting insects will be seen and taken by feeding fish. But this is when the fish will be looking for drifting insects so your chances of success are higher when insects are drifting.

See:

http://books.google.com/books?id=UUt... drift&f=false

http://books.google.com/books?id=wiT... drift&f=false

[tedshuck]

I read an article in the past couple of years (tried unsuccessfully to Google it) about how changes in tailwater releases on the Arkansas River in Colorado had a tremendous negative impact on the infamous Mothers' Day caddis hatch. My recollection is that it was thought some early water releases (very cold water) had altered the hatch cycles of the caddis reducing the quality of the hatch. So, it was posited that the timing of the water temperature changes, rather then the flooding/rising water, had reduced the hatches. This made sense to me since the river deals with the spring runoff annually. Having fished the Mothers' Day hatch for years, the quality of the hatches has been reduced considerably since about 2006 or so. I've heard from others that it might be improving.

I was told, earlier this year, that the recent poorer quality of the Mother's Day caddis hatch on the Arkansas was due to the overall health of the river improving. Prior to about 2000, the river was more polluted with mine tailings and the water flows were not as carefully managed. This made problems for stoneflies and mayflies, but caddis could still thrive. Since the water quality has improved, stoneflies and mayflies are more plentiful and the caddis have some competition.

Ted

[Byron haugh]

Tad,
Odd, my reading in the past was the opposite.....that tricoptera were most vulnerable to stream pollution.....?

Can you provide your source?

Thanks

[tedshuck]

Tad,
Odd, my reading in the past was the opposite.....that tricoptera were most vulnerable to stream pollution.....?

Can you provide your source?

Thanks

Byron,
My source was a fishing guide from the ArkAnglers fly shop in Salida, Bill Dobson.

Here is an article from Karl Blankenship where he states that caddis are more pollution tolerant than mayflies or stoneflies.
https://www.fish.state.pa.us/anglerb...9/cadisfly.htm

I have noticed that other streams in Colorado with mine pollution, heavy metals, in the water tend to be dominated by caddis. It may be that mayflies are more tolerant of different types of pollution.

Regards,
Ted