CLM20-2 full issue-1 - Flipbook - Page 33
VIEWPOINT: Dams without beavers: could beaver dam analogues yield benefits in the UK?
BDAs are simple and cheap to construct. Here, a row of wooden posts is being inserted into the bed of the
creek in order to secure the BDA. USFWS Pacific Southwest Region
The streams were no longer connected to the
floodplain, and the fertile valley bottoms dried
out. In the USA, therefore, the role of beavers
in the creation of complex fluvial systems
was obvious, leading to recognition of stream
evolution as an eco-geomorphic process; one that
is not only influenced by physical features, such
as gradient and flow, but also by living organisms,
such as beavers and plants.
BDAs in the USA
In the 1970s and 1980s, the restoration of river
systems in the USA was deemed necessary in order
to reverse declines in salmonid populations, which
led to the idea of imitating the actions of beavers in
places where they were scarce or absent, or where
creeks were so heavily degraded that reintroduction
of the animals was not an option. As a result, the
ecologist Michael Pollock developed the concept of
and construction methods for BDAs, and pioneered
their use as a cheap, relatively simple technique
(Pollock et al. 2015).
The BDA movement began in Bridge Creek,
Oregon, in 2007, when Pollock and his colleagues
attempted to strengthen existing beaver dams
with wooden posts in order to prolong their lives.
Subsequent work revealed a positive correlation
between salmon populations and the presence of
these dams, which led to the initiation of a more
ambitious programme of building structures
that imitated beaver dams along a 3.4km stretch
of Bridge Creek (Pollock et al. 2014). Initially
76 BDAs were constructed, with a further 45
added between 2010 and 2012. In the four years
after BDAs were first installed, the production
(population density, growth and survival) of
young steelhead salmon rose by approximately
175% (Bouwes et al. 2016).
Following the success of the Bridge Creek trial,
BDAs have become probably the most widespread
tool in the restoration of river systems throughout
the USA. BDAs offer a number of advantages
over other hard-engineering options: they are
simple and cheap to construct, ideally suited to
being built by volunteer labour; they use materials
that are readily available on site; and they are
generally free of bureaucratic constraints, such as
planning regulations, because they are essentially
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