Considering the Ecological Impacts of Dam Removal

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Considering the Ecological Impacts of Dam Removal

• Stream ecosystem response to small dam
  removalLessons from the Heartland
• The Conceptual Basis for Ecological Responses to
  Dam Removal
• Roslyn Odum BEE 694 EcoHydrology March 2,2006

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Yer Basic Rundown

• Overview of Removal
• Why Remove?
• Ecological Responses to Removal
• Measurement Methodology
• Conclusions and Discussion

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Overview of Dam Removal

• Our primary goal is to examine how geomorphic
  forms and processes affect stream ecosystems
  across a range of trophic levels. (Doyle 2002)
• -- Small dams dominate the waterways of Wisconsin
• -- Pervasive decline in the structural integrity
  of dams inspires initial wave of removals as a
  safety and management concern1960s
• -- 500 of 75,000 dams in U.S. have been removed

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Overview of Dam Removal

• - Limited understanding of the geomorphic and
  ecological effects
• Sheer abundance
• Structural and functional diversity
• Debate big ones, extract small ones
• Small dams ? minimal impacts

• Trends in the research

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Overview of Dam Removal

• We DO know a bit about geomorphic impacts of dam
  construction and operation as dams lead to
• SEDIMENT STORAGE UPSTREAM
• DEGREDATION DOWNSTREAM

• We DONT know much about geomorphic response to
  dam removal
• QUANTITY OF SEDIMENT STORED IN RESERVOIR
• SEDIMENT TEXTURE
• SPACIAL SCALE OF ADJUSTMENTS
• POTENTIAL FOR EPISODIC FLOOD EROSION OF HIGH
  TERRACES FORMED FROM DEEP PRE-REMOVAL SEDIMENT

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Overview of Dam Removal
STAGE A Pre-removal STAGE B Lowered water
surface STAGE C Degradation STAGE D Degradation
and widening STAGE E Aggradation and
widening STAGE F Quasi-equilibrium
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Why Remove?

• Fish that must pass through a sequence of 5 dams
  with 90 success of passage through each face a
  net loss of 41 of the original number attempting
  to migrate.

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Why Remove?

• Restoration of Hydrologic Regimes
• Geomorphic effects of trapping sediment behind
  dam
• 50 of channel complexity has been reduced
  through active channel alteration, bank
  hardening, and hydrologic alteration through
  flood control

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Removal Responses

• The Big Five
• Riparian vegetation
• Fish
• Macroinvertebrates
• Unionid mussels
• Nutrient dynamics

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Removal Responses

• RIPARIAN VEGETATION
• Measurement Methodology survey of 13 sites from
  Wisconsin that represented a range of years (1
  30) since removal (in place of one site over a
  number of years)
• Initial vegetation in impoundments dominated by
  weedy plants, eventually succeeded by slower
  growers with lower seed production and less
  effective dispersal mechanisms (ie. riparian
  trees)
• Bare sediment is extremely rare
• Species diversity is highly variable among 10
  year post-removal sites

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Removal Responses

• FISH
• Measurement Methodology 5 study reaches surveyed
  to estimate habitat characteristics (e.g. riffle
  occurrence, cover for fish, substrate type),
  relative abundance, and size of fish once per
  year

• Restoration of fish populations has been one of
  the most common arguments made in support of dam
  removal
• Recovery of fish species upstream of removed dam
  (read migration barrier) expected
• Absence of fish populations in vicinity of dam
  also due to habitat limitations (temperature,
  flow patterns, flood dynamics, predators)

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Removal Responses

• MACROINVERTEBRATES
• Measurement Methodology surveyed cross-sections
  and collected benthic macroinvertebrate samples
  in 6 reaches before and after the removal of two
  dams to consider the rate of change and the
  relationship between macroinvertebrate assemblage
  structure to habitat change.
• Central role in food webs
• Relative mobility and direct association with bed
  substrate reflect local conditions in stream over
  timerapid response to change in geomorphic
  conditions

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Removal Responses

• UNIONID MUSSELS
• Measurement Methodology post-removal survey of
  mussels within impoundment and downstream of
  single dam removal site
• Mussels are one of the most threatened groups of
  aquatic species in the United States?70 of 300
  native species are endangered
• Mortality rates following dam removal 95
• Much slower recovery than fish and
  macroinvertebratesmussels are long-lived slow
  growing organisms
• Fish colonization in new habitat is important as
  mussel larvae disperse to new sites by
  piggy-backing on fish

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Removal ResponseUNIONID MUSSELS
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Removal Responses

• NUTRIENT DYNAMICS
• Measurement Methodology 5 time periods
  representing five geomorphically different
  conditions were modeled assuming steady-state
  nutrient uptake parameters to examine the
  retention of soluble reactive phosphorus
• The most fundamental level of ecological change
  in rivers is the community of algae, bacteria,
  and other microbes whose growth can be linked to
  nutrient retention in the streamand who fuel
  most river food webs!
• Simulation shows that pre-removal stagnant
  backwater conditions greatly enhance nutrient
  retention (downstream reduction in nutrient
  concentration)
• Model shows that changes in channel shape can
  influence nutrient retention

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Removal ResponsesNUTRIENT DYNAMICS
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Conclusions
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Conclusions
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Conclusions

• a dam removal site may be perceived as a
  ecological failures simply because the benefits
  of removal have yet to be realized.
• PERIPHYTON recover rapidly and move towards
  equilibrium with channel morphology
• FISH/MACROINVERTEBRATES decline initially due to
  habitat disturbancerequire some vegetation
  recovery to provide habitat, shading, and organic
  matter
• VEGETATION show variable patterns over decades
• MUSSELS impacted most severely and recover over
  decades

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Discussion

• Which dams should be considered for removal?
• What realistic goals should management agencies
  develop for dam removal?

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Discussion

• Science and Policy reluctance towards
  deconstruction
• Social actions that dictate ecological responses
  hydrologic alteration, water diversion, bank
  hardening, land use conversion, exotic species
  introduction, water quality impairment (clarity),
  seasonal patterns of discharge ?all of these
  complicate recovery of indigenous species

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Discussion

• What?