Size-dependent effects of dams on river ecosystems and implications for dam removal outcomes

Understanding the relationship between a dam’s size and its ecological
effects is important for prioritization of river restoration efforts based
on dam removal. Although much is known about the effects of large storage
dams, this information may not be applicable to small dams, which
represent the vast majority of dams being considered for removal. To
better understand how dam effects vary with size, we conducted a
multidisciplinary study of the downstream effect of dams on a range of
ecological characteristics including geomorphology, water chemistry,
periphyton, riparian vegetation, benthic macroinvertebrates and fish. We
related dam size variables to the downstream-upstream fractional
difference in measured ecological characteristics for 16 dams in the
mid-Atlantic region ranging from 0.9 to 57 m high, with hydraulic
residence times (HRT) ranging from 30 minutes to 1.5 years. For a range of
physical attributes, larger dams had larger effects. For example, the
water surface width below dams was greater below large dams. In contrast,
there was no effect of dam size on sediment grain size (though the
fraction of fine-grained bed material was lower below dams independently
of dam size). Larger dams tended to reduce water quality more, with
decreased downstream DO and increased temperature. Larger dams decreased
inorganic nutrients (N, P, Silicate), but increased particulate nutrients
(N, P) in downstream reaches. Aquatic organisms tended to have greater
dissimilarity in species composition below larger dams (for fish and
periphyton), lower taxonomic diversity (for macroinvertebrates), and
greater pollution tolerance (for periphyton and macroinvertebrates).
Plants responded differently below large and small dams, with fewer
invasive species below large dams, but more below small dams. Overall,
these results demonstrate that larger dams have much greater impact on the
ecosystem components we measured, and hence their removal has the greatest
potential for restoring river ecosystems.