What do we mean by structures? That’s a good question, and an understandable source of
concern for an environmental community that has been working so hard to remove
barriers in streams. One reviewer
of our original concept noted that:
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“I actually think this may be interesting in the
location they are talking about [small headwater streams] but am a little
concerned about the perception and implementation of a larger program. We
have been working really hard to REMOVE blockages to fish migration, not
construct them. In the headwaters of the Potomac, it may be fine since
anadromous fish would have a hard time getting over Great Falls anyway, but
other places may not be good.” |
Since our project represents an ecologically guided attempt to restore
the landscape storage of water that was once a
ubiquitous component of the ecosystem created by the headwater beaver dam -
why not use (and import) beavers to do the job? Several of our project
reviewers said as much, and clearly beavers would provide a more ecologically
rounded service. However, the best response to importing beavers to do the
work comes from the other reviewers who said - emphatically - "you aren't
planning to use beavers are you!" Or perhaps more colorfully in a recent
David Heishman editorial in the Moorefield Examiner (9/10/2003) where he noted:
"Beaver dams = Damn beavers. I first heard about needing more beavers
several years ago when wetland preservation was front burner in Washington tree
hugger circles. ... My practical experience with flat tailed, sharp
toothed, beady eyed creatures, both bureaucratic and natural led me to oppose
the importation of either."
While CI agrees that beavers would "do it better", the reality is that in the majority of
areas where we might want to work the concept of importing beavers would
be only slightly less palatable than importing wolves to control the deer
herds. And, on a more practical level, this project seeks to
quantitatively demonstrate that a series of small structures along headwater
streams can provide water resource
conservation that cannot be achieved in any other way, and at a water
storage cost that is extremely competitive with other methods. Ultimately,
this may increase
understanding of and, hopefully, tolerance for beavers but, regardless,
such a
demonstration will be best achieved using a system that we can control
(and that most people will tolerate), which will require using man-made structures of some kind.
On to the structures themselves. Beaver
dams come in two basic sizes. There
is the huge, valley-wide structure like the one shown below at Short Mountain in
Hampshire County. That line of shrubs to
the right is the beaver dam, now well anchored with vegetation.
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Then
there is the bank-to-bank variety, like those below Sleepy Creek Reservoir (at
least they were there before Hurricane Isabel came through).
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The
bank-to-bank beaver dam provides the hydrological context for our project. When built by beavers, they
run pretty much straight across the stream, and are often started by felling a
sizeable tree. They have a tendency to 'blow-out' on the sides and, as
might be apparent from the above picture at right, can exacerbate erosive forces
along the banks. As far as the beavers are concerned these are not
problems. Blowing out at the margins tends to increase the width of the
stream, hence the size of the pool, and the beavers don't seem to mind doing
repair work. Its what they do after all.
However,
not being extremely industrious rodents with unlimited time on our hands, the
staff of Cacapon Institute has no intention of trying to create structures
that require regular maintenance activity - like beaver dams. We also
don't want to create barriers to fish and invertebrate movement, increase
erosive forces on the stream banks, or prevent the transport of sediment -
something that streams are 'supposed' to do. We simply want to raise the
level of the stream so that water will seep into the banks more readily at many
conditions of flow. The structures
will need to be very low maintenance and inexpensive to install.
It
turns out that this is a good time to be considering how to build such a
structure, as the emerging science of natural stream restoration has developed
structures that accomplish very much what we want to do. One class of such
structures is essentially a V-shaped weir, oriented so that the point of the V
faces upstream, and constructed
bank-to-bank, sloping down from each side toward the center. When
constructed correctly, erosive forces are focused on the center of the stream
channel, not the banks. This often creates a plunge pool - excellent fish
habitat - and does not erode the stream banks if installed correctly.
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Above are two examples of cross vane structures
from the Big Bear Creek, PA stream restoration project, perhaps the
largest natural stream restoration project in the East so far. The
structure on the left is made from quarried sandstone, the structure on
the right is made from hemlock. Both have withstood many high water
events without significant damage, including the the second largest twenty-four
hour rain fall event in their recorded weather history from Hurricane Ivan
in September 2004. While Big Bear Creek is much
larger than the streams CI is considering for the stream flow
restoration project, the same principles will generally apply.
Many thanks to Bill Worobec for inviting CI to tour this site, and for
sharing his experiences in bringing this project to reality. |
We
selected a variant of the cross vane, per the Big Bear log structure
above. The following graphic shows the basic design, which has to be
adapted to the unique characteristics of each structure site. As of
November 2004, 15 structures have been installed in the Site 1 meadow
- which offers interesting design challenges in creating stable
structures. The stream is very tiny, meanders constantly, has
deeply undercut banks just about everywhere (natural for this kind of
channel), blowouts, bank calving, much multiflora rose, and often has
orphaned "islands" of sedges in the middle of the stream.
Click
here to see the structures that have been installed thus far. |
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