Millponds: Nostalgic or Obsolete?

Millponds: Nostalgic or Obsolete?

 

A few decades ago millponds and millstreams were subjects for painters of bucolic landscapes. They also were power sources vital to local economies. Over time they have lost their economic role and their beauty is stained by their destruction of valuable habitats.

Most of the small dams holding back ponds in our watersheds allow water to escape over the top of the dam. That water was the top water of the pond – the water that was warmed by the sun. The heated water flows downstream warming everything in its path. This destroys all the coldwater habitats of the stream. Fish, such as brook trout, and especially their eggs and young no longer can survive without those coldwater habitats. Warmwater species from elsewhere, such as brown trout, take over, often with our help. As the original environment is further distorted, it may become preferred habitat for ecologically less appropriate species such as smallmouth bass and crappie.

For several streams flowing into Lake Ontario, species integral to the original ecosystem have been pushed out by the many little mill dams favoured by the human view of progress held a few decades ago. The Atlantic salmon population that was “landlocked” in Lake Ontario survived by migrating upstream to spawn and start their young*. Both the destruction of coldwater habitats by surface overflow from millponds and the barriers presented by the dams themselves barred the salmon from streams that had supported their population for glacial time.

Although less discussed by European settlers, American eels were even more important than salmon to first nations in the watersheds draining into Lake Ontario. These fish, not to be confused with lampreys, gather in the Sargasso Sea off Bermuda for mating. Their transparent young (elvers) migrate to the streams of the continents and up those streams to mature over a few years, before migrating back in the Sargasso to mate. Mature eels, trapped by dammed rivers, have been caught in lakes in the Salmon River watershed in the current decade.

Surface overflow dams affect not only sports fishes and eels. All invertebrates at the base of the food chain, that require cold water habitats, also are pushed out.

In addition to the barrier effects and the warm water effects, surface overflow dams also change the sediment flow downstream. As the water flow slows above the dam, silt, sand, gravel and all heavier particles carried by the water settle out and become sediment in the bottom of the pond. Not only does the pond fill up, but those settled particles don’t go downstream to make up the sediments required by everything from fish eggs to mayflies.

Unless one life-stage can fly, the invertebrates as well as the fishes have their gene pool along the stream broken into isolated segments. The basic processes of evolution are impacted by this fragmentation of the streams, and the fragmentation of gene pools, by dams.

In many cases, these old dams are relicts of the past that have lost their reason for being. Their ecological effects are still causing impacts and prevent restoration of habitats critical to the survival of species. These old dams restrain the possibilities of rebuilding the faunal structure of our Lake Ontario streams and the Great Lakes.

 

* Dymond, J.R. 1965. The Lake Ontario Salmon (Salmo salar) , Ed. H.H. Mackay. Ontario Ministry of Lands and Forests,141 pp.

 

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