Exploring the causes of flow attenuation at a beaver dam sequence.
Beavers influence hydrology by constructing woody dams. Using a before
after control impact experimental design, we quantified the effects of a
beaver dam sequence on the flow regime of a stream in SW England.
Building upon our previous research (Puttock et al., 2021), we consider
the mechanisms that underpin flow attenuation in beaver wetlands.
Rainfall-driven hydrological events were extracted between 2009 and
2020, for the impacted (n=612) and control (n=634) catchments, capturing
events seven years before and three years after beaver occupancy, at the
impacted site. General additive models were used to describe average
hydrograph geometry across all events. After beaver occupancy, Lag times
increased by 55.9% and declined by 17.5% in impacted and control
catchments, respectively. Flow duration curve analysis showed a larger
reduction in frequency of high flows, following beaver dam construction,
with declines of Q5 exceedance levels of 33% and 15% for impact and
control catchments, respectively. Using event total rainfall to predict
peak flow, five generalised linear models were fitted to test the
hypothesis that beaver dams attenuate flow, to a greater degree, with
larger storm magnitude. The best performing model showed we can have
high confidence that beaver dams attenuated peak flows, with increasing
magnitude, up to between 0.5-2.5 m 3 s
-1 for the 94 th percentile of event
total rainfall; but we cannot confidently detect attenuation beyond the
97 th percentile. Increasing flow attenuation, with
event magnitude, is attributed to transient floodplain storage in low
gradient/profile floodplain valleys. These findings support the
assertion that beaver dams restore attenuated flows. However, with
long-term datasets of extreme hydrological events lacking, it is
challenging to predict the effect of beaver dams during extreme events
with high precision. Beaver dams will have spatially variable impacts on
hydrological processes, requiring further investigation to quantify
responses to dams across differing landscapes and scales.