Application of fallout radionuclides as indicators of eco-geomorphic adjustments to dams
Abstract
Because of the profound effects of dams on flow variability and sediment transport dynamics, extensive research by both geomorphologists and ecologists has attempted to document the array of bio-physical changes associated with dams. Yet specifically linking ecological responses to geomorphic processes has proven difficult and thus is often lacking in eco-geomorphic assessments, indicating the need for a broader array of efficient methodological approaches. This research investigates new approaches for assessing bio- physical changes occurring both in-channel and across the broader riparian zone as a consequence of reduced flows downstream of flood control structures in the Upper Connecticut River watershed. In particular, we seek to ascertain the role of tributaries in either exacerbating or attenuating the effects of dammed mainstems. Our approach takes advantage of the naturally occurring short-lived fallout radionuclides 7-Be and 210-Pb by documenting changes in radionuclide activities and inventories as a functions of in-channel sediment residence time and floodplain overbank inundation histories. As most streams in the Upper Connecticut River watershed have limited fines, we ran controlled exposure experiments to establish whether the more common sand-sized grains can sorb radionuclides and be used as tracers. Results indicate that even profiles containing well sorted 2-4 mm grains develop a classic exponential decay in 7-Be activity with depth, with surface samples (0-2 mm depth) having very high 7-Be content >20 Bq/kg) that can be modeled to approach equilibrium with the atmosphere. To capture compositional effects we compared the grain surface area normalized 7-Be activity to the ratio of 7-Be to excess 210-Pb and to the ratio of 7-Be to total 210-Pb. We find that the ratio of 7-Be to total 210-Pb has the strongest relationship with grain surface area normalized 7-Be activity (r = 0.90; p < 0.01). Analyses of 7-Be activity of transitional bed load sediment in the watershed reveals that the activity is highly seasonal and reflects both variations in activity of sediment sources and limited sediment residence time within tributary junctions. In these same systems benthic communities also exhibit a strong seasonal variability. In spring, neither the 7-Be activity of the sediment, nor benthic communities, exhibit clear relationships with sample site location. In contrast, during the late summer the ratio of Ephemeroptera (mayflies)/Trichoptera (caddisflies) decreased significantly below tributary junctions. This decrease in benthic community ratio was driven by increases in caddisfly abundance and was strongly correlated with the presence of recently 7-Be tagged transitional bedload sediment (r = 0.94; p < 0.001). Large inventories of excess 210-Pb across floodplain surfaces, in combination with post- dam establishment and dominance of more upland tree species, indicate the lack of inundation disturbance of previously flooded surfaces.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.H33B1004M
- Keywords:
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- 0454 Isotopic composition and chemistry (1041;
- 4870);
- 0481 Restoration;
- 1808 Dams;
- 1825 Geomorphology: fluvial (1625);
- 1834 Human impacts