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Date: | Wed, 25 Apr 2007 13:22:10 -0400 |
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Meli-
For many years when my son was growing up with his mother in
Connecticut, I travelled back and forth from Albany to Norwich every
other weekend. The route went past the West Branch Reservoir on the
Colebrook River. Many times over the years the lake was drawn down far
enough to allow motorists to cross the bottom on Old Rte. 8, and
needless to say, my son and I spent a fair amount of time exploring. The
steep slopes were clearly scoured clean of topsoil, but the level areas
appeared to retain the original soil surface under deposits of mud, sand
or till, just as you describe. However, it seemed to me that the
erosional process is not simply a "bathtub effect." A lake shore is
basically a beach, subjected to the specific types of erosion one finds
on beaches: Longshore currents and wave action. The area of erosion is
basically the line of contact between the lake surface and the shore.
Under normal circumstances waves erode sand or soil from a beach to be
carried by the longshore current and redeposited further along the
beach. When a lake is repeatedly drained, this erosional interface moves
down the slopes relatively slowly allowing prolonged exposure to wave
action but carrying the eroded soil down until the lake level stops
receding. The effect is that all the soil gets deposited in low-lying
areas, leaving the slopes stripped. It is areas subjected to prolonged
beach currents and wave action (steep areas) that are scoured. Areas
that are level are covered and uncovered too quickly to allow this type
of erosion.
Batchellerville, N.Y. is a town that was partially submerged during the
creation of Great Sacandaga Lake ca. 1911. When I did a small survey
there a couple of years back, the lake was lower than it had been since
the 1960s. It was clearly visible that the steep slopes were seriously
eroded by the lake's annual drawdown. However, despite the fact that the
level areas I tested were normally in a shallow beach environment, which
should have been scoured pretty clean, they had intact topsoils buried
by sand and gravel much like a lake-bottom site. The adjacent large
bridge abutment had acted like a breakwater, protecting the site from
erosion by interrupting the longshore current and reducing wave action.
This allowed deposition instead of scouring in the lee of the bridge,
and the site was preserved.
Martin Pickands
New York State Museum
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