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.