Dam removal, hydrogeology, and biological systems: Data from a freshwater, tidal system

Background/Question/Methods

Naturally functioning aquatic systems provide many ecological and human benefits.  Restoration of these environments has a high potential to improve these ecological and human benefits.  Unlike large restoration projects, many small scale projects are not well documented.  Without monitoring, we cannot assess the outcome of these restorations.  Dam removal sets in motion physical and chemical changes above and below the dam site.  Many of these changes result in ecological changes.  The Nature Conservancy will remove two manmade dams on a small watershed flowing into a freshwater tidal stream, Holts Creek.  Their goal is restoration of 2,000 linear feet of stream channel with edging wetlands and restoration of connectivity between the tidal stream and ponds upstream of the dams.  Our short term goal is to document the physical and ecological state of this system so we might understand changes after dam removal.  Sediment, plant, invertebrate, and fish samples were taken in the ponds above the dam and in Holts Creek, below the dam, and both upstream and downstream of the mouth of the dam, in Holts Creek.  Upstream sampling will serve as control samples, not directly impacted by dam removal.  Pb-210 analysis provided estimates of sediment accumulation rates and potential sediment flux.  Ponar dredge samples provided sediment invertebrate samples.  1-m² quadrat samples were used to estimate plant diversity and abundance on pond edges above the dams.
Results/Conclusions
The sediment accumulation rate was 0.385 g cm^-2 yr^-1 in the pond above the dam.  Bottom samples showed significantly lower invertebrate species richness in the pond above the dam than below the dam in Holts Creek.  Invertebrate species richness was similar in Holts Creek above and below the mouth of the dam.  Riparian vegetation richness ranged from 8 - 12 species per m² at 3 sample times.  Two non-native species, Murdannia keisak and Microstegium vimineum were among the most abundant riparian species.  During the summer of 2009, non-native species covered, on average, 35% of quadrats and native species covered 14%.  These data provide the context in which to interpret physical and ecological changes after dam removal.  Sediments that are currently deposited above the dam will likely be deposited in Holts Creek.  The invertebrate richness data suggest we may see an increase in richness above the dam after connectivity is restored.  The plant data suggest that non-native species will be a concern when dam removal opens sediments to colonization.