Adrienne M. Gibson ACF Abstract FY11

"Low-Head Dam Removal Causes Immediate Physical Habitat and Water Chemistry Degradation"

Aquatic Science

Adrienne M. Gibson

This two year study focused on understanding the effects of the removal of
the low head Nashville dam on the Thornapple River in Barry County, MI.  A
comparison of data taken from pre to post dam removal indicated changes to
the physical habitat, as well as water chemistry.  In the reach directly
below the dam there was a 26 cm average cross-sectional channel aggradation
 increase in fine sediment from pre- to post-dam conditions, indicating
reservoir erosion and subsequent downstream deposition.  This was in
contrast to relatively minor changes in cross-sectional profile observed
throughout the other study reaches, both up and down stream of the former
dam.  In addition, there still appears to be a turbidity signature directly
below the dam in response to increasing discharge.  Specifically, at the
site directly downstream of the former reservoir, we observed a 0.5 to
1.0-fold elevation in turbidity (NTU) during the rising limb of a flood
event above that observed at other measurement locations further down and up
stream.  Nutrient limiting conditions also changed from pre-and post dam
removal.  Pre dam removal nutrient levels showed above dam sites to be
phosphorus-limiting and the sites below the dam to be nitrogen limiting.  This
changed after the dam was removed, when all sites appeared to be phosphorus
limiting.  The most drastic nutrient limiting change occurred directly above
the former dam.  The pre-dam removal N/P ratio of 22.00 was found reduced to
7.90 after dam removal.   Results indicate that conditions 1-year post
removal are still degraded, but that these impacts are quite proximate to
the former dam site.
 

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