Pondology
news from the waterfront by Tim Matson

PONDS AT YEAR'S END: Koi, Stream Pools, Rusty Pipes, Otters, and Drought

Looking back over the pond season now closing out, I recall a few situations worth a mention. Perhaps the biggest surprise was a couple of ponds stocked with koi. Not long ago the only fish Vermonters stocked were trout. True, two ponds full of koi don't make a trend, but for those contemplating koi let me put out a caution. Technically, stocking koi in Vermont is illegal, except in a closed pond, meaning one where the koi cannot leave the pond and possibly breed in our streams, where trout are preferred. Of course even a pond with a closed spillway might be subject to flooding, with a few koi surfing away in the waves.

I know that brightly colored koi are attractive, but the irony is that being a vegetarian carp, koi like to root around in the pond bed, and may stir up enough sediment to cloud a pond so thick the koi  become invisible. The degree of koi cloudiness may depend on the pond soil. One of the koi ponds I saw had beautifully clear water and brilliant fish, probably due to a nice hard pan pond bottom resistant to stirring up. The other pond had been lined with native clay as a sealant which stirred up easily. It took handfuls of fish food to temp the koi to the surface where they showed off briefly, ate, and disappeared again. The owner and I disagreed about whether the turbidity was due to the koi, the aeration system, or algae. Or all three. I was betting on the koi. I see now why koi put on such a pretty show in ponds lined with a plastic membrane: no soil to stir up.

It was an unusually dry summer, so I saw a number of ponds with low water levels, often accompanied by blooms of algae. In some cases they were relatively new ponds, and the low water may have been due in part to pervious disturbed pond bed soil, which hadn't had time to seal naturally with sediment. But in several of these low water ponds, it turned out that test pits hadn't been dug prior to building, and a reliable water supply confirmed. The task then became finding a new water supply, whether from roof runoff, hillside drainage, springs, or a well. In the direst circumstance, low water ponds might require a membrane liner.

Problems common to aging ponds appeared predictably, especially as the ponds built thirty or so years ago age out. Steel spillway pipes are reaching the end of their lifespans, and I found several standpipes corroded and leaking. Sometimes it was the inlet rim disintegrating and consequently lowering the waterlevel -- and raising the suspense factor about a complete spillway failure. One pipe started leaking at the bottom of the standpipe, where the drain and discharge connected. Same result: pond level down. Same solution options: repair or replace the spillway pipe, or switch to an earthen overflow channel.

I also saw an old plastic spillway pipe installed culvert syle, which had cracked and was leaking, taking the pond level down. The repair for this pipe would offer the owners a chance to lower the water level even further and do some excavator edge work to reclaim the pond edges from invasive plants which had grown in over the years. For aging ponds, keep an eye on pipe spillways. And remember that drawdowns for spillway repairs can offer an opportunity for other repairs done best with lowered water.

One not so common sight was a traditional stream ponding technique. I found it at the site of a culvert replacement, where a rusting steel pipe was replaced with a concrete box culvert. A stream pool below the pipe had been flattened out to allow for better fish travel. Neighbors complained that the pool was a popular swimming and fishing hole, and they were not happy to see it disappear. So the contractor installed three granite blocks in the stream bed. They were set at a level high enough to create a small waterfall in the stream, but not enough to block fish traffic. After several weeks, water cascading over the blocks had carved out a new pool to replace the one lost to culvert repair. A similar "digger pond" technique is described in two of my books: the Earth Ponds Sourcebook, and Earth Ponds: The Country Pond Maker's Guide. I was visiting the pool the other day to watch its deepening process, and scared off an otter. The otter shot out of the pool and swam fast downstream, probably with a belly full of brook trout. The neighbors aren't the only ones happy about the restored pool.

Lest I convey the wrong impression about the ponds I visited over the summer, most were attractive, happily used, and a highlight of each property. I offer the thoughts above only to illustrate a few of the troubles and oddities that can occur, and possible solutions. Most pond owners love their ponds, warts and all. Or is that toads?

December 1, 2015

 

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