CLM20-2 full issue-1 - Flipbook - Page 34
VIEWPOINT: Dams without beavers: could beaver dam analogues yield benefits in the UK?
models or powered by the
hydraulic system of a small
excavator or tractor. Branches
of riparian trees such as alder or
willow are then woven between
the posts. The dam is usually
strengthened by placing rocks
and stones along the bottom to
prevent scouring underneath,
and mud and vegetation are
then plastered against the
upstream face. These dams are
deliberately made somewhat
porous in order to allow the
passage of fish.
In the USA BDAs are often
adopted by beavers, who
undertake their upkeep.
Without these animals they
will need a certain amount of
maintenance, which should
ideally imitate the actions of
beavers, such as sealing with a
mixture of mud and vegetation
– beaver dams are dynamic
systems in which modification,
decay and repair are normal
and natural.
Like natural beaver dams,
BDAs collect sediment and
raise the level of the riverbed
upstream, helping to rewet
Once the location of a BDA has been decided, mud, bed material,
meadows along the creek sides
turf and sticks of different sizes are used to build the first layer to
and to enhance important
just above the existing water level (top). The layers of the BDA are
habitats. Complexity can be
built up slowly to the desired height with branches (placed parallel
achieved by building dams
to the flow) and other material (middle), with the crest kept level in
order to avoid preferential flow or the creation of a weir. Untreated
level with the banks, which
wooden posts can be used to reinforce the BDA if needed
creates wetlands upstream,
(bottom), particularly in areas of fast flow. Utah State University
or by adding partial barriers
Restoration Consortium (CC BY 4.0)
which divert the water against
temporary structures. The construction of a BDA
the creek bank, causing erosion of the bank and
involves inserting a row of wooden posts, or in
encouraging the development of a more sinuous
some cases two parallel rows, into the bed of
course. Use of levelling instruments will show the
the stream or creek. Cheap, untreated softwood,
size of pond that will be created by a particular
which is fairly resistant to rot when submerged,
height of dam, allowing BDAs to be positioned
can be used, as can hardwood (e.g. oak or sweet
and designed in a way that minimises conflict
chestnut in the UK). Tanalised or otherwise
with neighbouring land users.
treated wood should be avoided. Posts are driven
BDAs are best built in series. They are often
deep into the streambed using post rammers,
stronger than genuine beaver dams because they
which can either be self-contained petrol-driven
are secured with embedded wooden posts, but
32 Conservation Land Management Summer 2022 | Vol. 20 No. 2
