DENIL FISH LADDER
USDI NPS ROCK CREEK PARK
Washington, DC
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Interpretive sign in the parking lot. ESA performed the geomorphology
design portions of this project.
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From downstream to upstream. Peirce Mill dam was necessary to force water
into the historic mill race, which allowed the hydrology to drive the grist
mill.
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Upstream intake. |
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Look inside on the left and you can see a glass observation window. If you
open a man-hole cover on the left you can climb down and into an underground
concrete vault and look out the window. I took the photo during a drought,
where no water was even flowing over the dam. During normal spring
migration of anadromous fisheries, the window would be underwater, so that you
could actually observe fish using the ladder.
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The denil ladder are a series of stairs (baffles). As the fish work
upstream (center of the weirs) they can rest in the slack water of the wings on
the sides of the ladder. The ladder creates highly oxygenated water, which
is the necessary 'attraction' water migrating fish key-in on as they work
upstream. The dam has been a long term fisheries (shad, herring, alewife
and perch) impediment.
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Bill Yeaman, the Natural Resources Management, US Park Ranger stated that
the ladder design requires on-going maintenance in the form of removing
flotsam debris that can create blockages. The design allows for the
over-top, walking gates (photos 2 and 3) to be lifted to allow the cleaning
of the stairs (notched weirs/baffles). You can actually see branches
partially blocking a weir step in photo #4.
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This ladder design allows the historic mill dam to remain in place at
the functioning mill,
while allowing fish passage. Alternative design could have been a
series of rock step pools
that would have better mocked piedmont
outcropping,
but would have compromised the cultural resources of
the site.
Submitted by Mark Burchick
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