Bog and lake drainage

oceanblue

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A substantial artificial lake (5 hectares) with about 1 Hectare of acid upland marsh at its upstream end which is a good habitat for palmate newts and common frogs needs repair work to the dam. (Location not far from my home in the Brecon Beacons but like field herping accounts I wish to be vague)

A substantial temporary lowering of water level is inevitable and after the works the restored lake may be smaller and the water table lower.

I gather the bog area will partly dry out and re-establish at about the same size or larger around the reduced lake.

My guess is November/December/January is the least disturbing time to perform major works for amphibians. There are not too many winter migrant birds to consider.

Is this timing the least harmful? Doing nothing is not an option.
 

Kaysie

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I think that as long as its filled back in before spring migrations, it would be the least impact.
 

Kaysie

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And just to be annoying, if it's in conjunction with a lake, the wetland is probably not a bog (which are completely fed by rainwater, and have no connection with groundwater), but is rather more likely to be a marsh.
 

oceanblue

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The lake is technically and legally a reservoir, probably built about 1830, but the owners have been trying to maintain the water level reasonably steady so it has a more natural look than water company reservoirs.

The work needs to be done because despite works to improve it recent flash floods have caused a revision in expectation of what may be a 400 year or 1000 year flood after a downpour on top of a municipal reservoir which began to crumble under the torrent (at Ulley) about a year ago.

The incoming water is from very small streams, the catchment area is about 10 times the size of the present lake and the water clear and oligotrophic. It is not stocked with fish and contains mainly minnows.

Marsh is the more accurate description of the upstream area at the moment.

Is work before November unwise, on the basis the palmate newts will still be in the lake and pools in the marshy area?
 

Kaysie

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It's hard for me to say, as I'm not there, and I don't know the historical hydrologic regime of the area, nor the natural history of the palmate newt.

I would say that when the newts would naturally leave the water (I'm assuming this is when it gets cold? or do they overwinter in the water?), that would be the ideal time to start lowering it. I imagine as the water level drops, the newts will head for winter hibernacula. Or they might all concentrate in the deeper water areas. You didn't plan on draining completely, correct?
 

Mark

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November should be fine. Larvae still in the pond in October are most likely to be overwintering. Your best window is between October and January. Adults will typically return in February but its weather dependant. Palmate larvae will overwinter but your main focus should be on protecting the adult population. You could always dip net the ponds in October and rescue a few stragglers before draining.

I love the Brecons – Being in Bristol it’s our closest area of wilderness and makes a great day out.
 

oceanblue

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Thanks a lot for the advice. There is always going to be a lot of water remaining even in the cut the size to below the reservoirs act scenario which is one possibility. How much will depend on the profile of the lake bed which is a GPS and plumb line job for next week.

Mark - if you want to look over the place I can PM you the location. It sometimes needs a key for the access road.
 

doktordoris

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Ive said it before on this forum, but Ill say it again!

Ponds drying out can be a good thing for herpts, many newt species can survive quite happily in the soggy mud at the bottom of a dried up pond, but the fish that predate them will all be killed.

A good state of affairs for the newts!
 

oceanblue

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The work on the outflow of the dam on this lake was completed a few weeks ago (early December 2010) and there is now a properly defined outflow channel which can cope with a maximum deluge over the catchment.

The water level is about 700mm lower, sufficient to prevent overtopping the dam from waves in a gale. The lake looks little changed with about 2M of extra land around most of the margin. I think it is unlikely the habitat has suffered much, I'll look for frogs and palmate newts in the spring. Thanks again to everybody who gave advice on timing.

I've attached two views, one of the outflow channel and a view up the lake, currently covered in ice.
 

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