What are the implications of a river in flood?

thehamo · 01-11-2011 3:52pm #1

What with the recent Dublin floods of late I got to thinking what are the implications on the fish stocks of the river? I know some fish may be able to ride it out but what happens to the smaller fish? Surely they couldn't survive it?

SeaFields · 01-11-2011 5:17pm

Interesting question.

I suppose for some of our species floods are very necessary to their life cycle on some rivers - spate rivers and salmon and sea trout. They are big, powerful fish though.

There are probably plenty of calm areas within the river too for fish to retreat once in flow becomes high. Gullies cut in the river bed and behind boulders and that. Also the areas that probably don't hold fish normally (long straight "dead" stretches) become another refuge for fish. You can pick up some lovely trout on a stretch of my local river during a flood that normally holds very little. Undercut banks too are favoured by fish during a flood.

As for fry I don't know but would love to hear if anyone else does?

fiacha · 01-11-2011 6:50pm

I was sitting down by the Tolka this morning thinking just the same thing. The pool I was sitting beside is back at normal levels, but during the flood it was approximately 4ft higher with a huge volume of water pushing through.

Good news is, the margins are full of fry. I didn't have my wellies on, so I couldn't get close enough to identify them

I have asked 5 different guys to move off over the last couple of weeks for fishing there out of season. There are people on this pool nearly every day of the week, and judging by the rubbish they leave behind some of them are cleaning fish on the bank. No doubt they will do more damage to the fish stocks this Winter than flooding.

coolwings · 01-11-2011 7:19pm

The silt of the summer is being washed away. Soon the gravel will be clean enough for the fish to lay their eggs in.
We need floods. Floods, and not the OPW, are what cleans out rivers best!

thehamo · 01-11-2011 7:46pm

Surely tho there's a difference between a flood and the absolute torrent of water that cause rivers to burst banks and so on? Looking at the tolka the other day my self, and videos of the dodder, the water would have washed a car away let alone a fish

ironbluedun · 01-11-2011 8:30pm

floods are no problem, its not floods that do the damage.

Zzippy · 01-11-2011 8:30pm

Even in high flood, there are plenty of hiding places out of the main current. The water velocity is actually lowest on the bottom, and increases as you go up in the water column, same as wind velocity increases higher up off the ground - drag effect and turbulence caused by rocks, stones etc, on the bottom...
In really exceptional floods, fish may seek shelter outside the usual river channel, when a river bursts its banks and spreads out on the floodplain. We found several big trout in a farmyard near the Clare River in November 2009, when this happened and there was major flooding. Once the water dropped again the fish were stranded.
As other posters have said, floods are essential - they serve to clear out the river of silt, improve spawning beds, allow fish to run obstructions like falls and weirs to access spawnign grounds upstream, carry silt to floodplains and improve farmland (in the absence of arterial drainage), and help to remind people that we are not all-powerful!

coolwings · 01-11-2011 8:52pm

EDIT: overlapping of posts here, and duplication of parts of the post with other very good answers by Zippy and Ironbluedun, but it refers to

Surely tho there's a difference between a flood and the absolute torrent of water that cause rivers to burst banks and so on? Looking at the tolka the other day my self, and videos of the dodder, the water would have washed a car away let alone a fish

Well I guess it depends on your view of what is happening.

If your house is on the bank, or your car driving along the inumdated road, then possibly "it bursts it's banks" according to your perception and goes where it doesn't belong.
But what it actually does is rise over it's low water banks and spreads out to find it's higher water banks, which are at the edge of the flood plain.
The stream is a channel within the wider channel which is the river. And the river is a channel which is zigzagging along within a even wider dry channel called the flood channel, or flood plain.

It is bottling it up that causes problems, compressing it, like if ignorant authorities build flood containment walls alongside the river, then it can't spread, the extra energy is focussed and concentrated, so it must speed up, and the hydraulic force of fast moving water is amazingly powerful, it can cut concrete bridges, and the like, damaging them.

Taking a long term river centric view, if you look at the river as the permanent item and all the other recent stuff as "clutter that doesn't belong there", then it is trying to wash away the obstructions so the flood plain is clear to take greater flows.

Another water expansion area that has been interfered with is the bog around the sources of the river. before they were drained these bogs filled up and absorbed water until they were merely solid looking, but actually over 90% water. Then they slowly released that water over the following months.
But the bogs were drained to make farmland, and get turf, and the absorbent sponge structure dries, collapses and ceases to function, so when it rains the water now goes stright into the river, all with a few hours. So the downland section, where the towns are built, gets it all at once. We are living in the environment our grandparents and parents made for us, both good and bad.

As for the fish, they do just fine. As the speed of the water increases, the turbulence at the edges also increases, and in that turbulence is slower water. The fish move into their cold water lies, shallower, very much closer to the edge, often where dry gravel is showing in low water, usually in little bay-lets, or the slower inside of bends, and they have a good feed on the live items going past in the drift.
Where it can spread over surrounding land, the fish move in and have a feast of drowned terrestrials like worms and insects, so winter does not have to be a lean time for them in the natural run of things.

Your concerns do apply in spades to the gorge type river sections. When contained by rock or concrete banks, there is a scouring effect from the high speed flows created, and food items are swept away. The result is that over the following weeks, the bigger fish will vacate the pools they used to live in and drop downstream looking for more fertile places to live.
Think of every large wild river fish you ever caught. Wasn't there cover from predators, adequate depth and food, and protection from excessive flows all present within 300 metres in that location?

thehamo · 01-11-2011 9:25pm

Excellent replies folks. Many thanks My worries have been alieved.

Another question that I was pondering. On a river such as the tolka, which has suffered greatly due to pollution, would the flood waters wash away the pollutants, thus cleaning the river out, or would they become part of the fabric of the river and its banks thus having a negative affect for years?

*by the way, I know the tolka has become cleaner in recent years, I was just using it as an example as it has suffered greatly in recent years.

SeaFields · 01-11-2011 9:41pm

As coolwings said it not so much that the river flood the urban environment, its more of a case of the river tried to do what it has done for thousands of years except we have gotten in the way. Impermeable surfaces is one of the things we build close to rivers in the form of roads, footpaths, etc. Not only do they channel water more quickly into the streams causing more intense floods with less of a lag period from the start of it raining to the peak of the flood level, they also carry all the crap that has been left behind by us on the surface.

It has been shown to have negative consequences in other countries (more populated river basins than what we have here) but I don't know if it has been researched to much degree here.

ironbluedun · 02-11-2011 11:27am

thehamo wrote: »

would the flood waters wash away the pollutants, thus cleaning the river out, or would they become part of the fabric of the river and its banks thus having a negative affect for years?

i would imagine yes is the broad answer...at least in the short term. in my unscientific experience really low water conditions are bad for fish. just look at the UDN in salmon and other parasites and diseases, they thrived (still do) in low water more 'stagnant' conditions.
whenever i have seen salmon with UDN its always been during periods of low water or drought, but i have not seen much of UDN in recent years.

Zzippy · 02-11-2011 6:56pm

ironbluedun wrote: »

i would imagine yes is the broad answer...at least in the short term. in my unscientific experience really low water conditions are bad for fish. just look at the UDN in salmon and other parasites and diseases, they thrived (still do) in low water more 'stagnant' conditions.
whenever i have seen salmon with UDN its always been during periods of low water or drought, but i have not seen much of UDN in recent years.

That's most probably furunculosis, not UDN. An outbreak of UDN would be pretty serious, whereas furunc is a common enough fungal infection that is exacerbated by warm temperatures, often occurring during low flow, or when fish are stressed (also in low flow, or around spawning time). What a lot of people think is UDN is usually just furunc, not half as serious.

slowburner · 04-11-2011 12:50pm

Not really: as much silt will be washed into a river as will be washed out in a flood. The only way that a flood could clean out a river is if absolutely no suspended solids could be washed back in. To think that an increase in flow will clean out all silt is incorrect because heavy rain carries equal amounts of silt off the ground and back into the river. As the flood subsides, the rate of flow slows down and deposits silt again. This is one of the reasons farmers are not supposed to plough in the winter - ploughed soil allows considerably more soil particles to enter watercourses than fields left fallow.
As many pollutants will be washed in too as will be washed out, if they are in the catchment. As floodwaters rise, they can stir up settled deposits which may contain pollutants or other deleterious matter - this is the primary reason why angling is rarely good in a rising flood or at the height of a flood - suspended solids cause difficulties for all fishes' oxygen supply.

The reality is that whatever is on the ground will end up in the river - if it can be transported by water.

Catchments in urban areas will always be more vulnerable to rapid rises in water levels and pollutants because hard surfaces cause rapid runoff. And the water will be dirtier. In a catchment unaltered by man, the ground has the capacity to absorb more water and release it more gradually.
Dirty floods are really the product of man's interference with either the river itself or the catchment area. All rivers are subject to rapid rises when the water table is at its maximum.
Rivers tend to remain in equilibrium unless there are truly exceptional floods like the recent one in the Dublin area and hurricane Charlie, when rivers can change their course. If the pool/run sequence is altered under such circumstances, it may take some time for the river to return to a state of equilibrium, from an angler's point of view.

Another very negative factor concerning game fish in our rivers is when major flooding occurs. If there are significant floods after spawning (mid December onwards in most catchments) there is a strong risk that eggs and fry could be washed out if the floodwaters are sufficiently strong to disturb the spawning gravels.
In latter years, flooding at this time seems to have become more frequent and may contribute to an overall decline in salmon stocks in particular.

So on balance, are floods a good thing or a bad thing?
Neither, they are just a natural part of the river's being. Rivers always return to their state of equilibrium unless irrevocably altered by man or cataclysmic flooding.
By way of example, the recent flooding of Dundrum town centre was the river Slang attempting to revert to its natural course. Similarly with the Poddle and most, if not all of the culverted streams in the Dublin area.

ironbluedun · 04-11-2011 1:59pm

good post but silt and chemical pollution are different.

04-11-2011 5:40pm

slowburner wrote: »

So on balance, are floods a good thing or a bad thing?
Neither, they are just a natural part of the river's being

very true. A bit like wild fires.

slowburner · 04-11-2011 7:58pm

ironbluedun wrote: »

good post but silt and chemical pollution are different.

Very true. Silt, by and large, is the equivalent of clay on land - it is a conglomeration of the finest soil particles which are deposited in the slowest flowing areas of the river bed. The sad reality with silt in most of our rivers these days is that pollutants, chemical or organic tend to become bound to particles in the silt.

What are the implications of a river in flood?

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