Ecological significance of riverine gravel bars in regulated river reaches below dams
Abstract
A gravel bar has been recognized as ecologically significant in that they provide simplified habitat with topographical, hydrological and thermo-chemical diversity, while enhancing material exchanges as interfaces laterally between aquatic and terrestrial habitats, and vertically between surface and subsurface waters. During past several decades, regulated rivers below dams have been loss of a number of the geomorphological features due to sediment starvation by upstream dams, accompanied by a subsequent degradation of their ecological functions. Despite a growing concern for gravel bar management recognizing its importance in recovering riverine ecosystem services, the ecological roles of gravel bars have not been assessed enough from the empirical perspectives of habitat diversity and organic matter interactions. In this study, we investigate the 'natural filtering effects' for reducing lentic plankton and contaminants associated with self-purification, and 'physicochemical habitat complexity' of gravel bars, focusing on reach-scaled gravel bars in rivers located in three different countries; First is the Uji River in central Japan, where there has been a loss of gravel bars in the downstream reaches since an upstream dam was constructed in 1965; second is the Tagliamento River in northeast Italy, which shows morphologically intact braided bar channels by natural flooding events and sediment supply; third is the Trinity River in the United States (located in northern California), the site of ongoing restoration efforts for creating new gravel bars through gravel augmentation and channel rehabilitation activities. We traced the downstream changes in particulate organic matter (POM) trophic sources (composed of allochthonous terrestrial inputs, autochthonous instream production and lentic plankton from dam outflows) in order to evaluate the roles of the geomorphological features in tailwater ecosystem food-resources shifting. We calculated suspended POM concentration and benthic POM density, and analyzed stable isotope signatures (δ15N and δ13C) and C:N ratios to effectively identify the three sources and longitudinally estimate their relative contributions. Our results showed that the retention capacities of riffle-pool sequences in the Uji R. differ depending on their POM trophic source. The relative contribution of lentic plankton decreased steeply in riffles ranging 10 to 14%, whereas that of allochthonous particles decreased in pools accounting for 7%. The braided gravel bar in Tagliamento R. had both high POM retention and high algal production. We found an uneven distribution in the amount of POM along the bar shore, while its trophic source composition was highly heterogeneous. Moreover, such spatial variation was highly correlated with secondary flow velocity and direction. In addition, physicochemical differences (e.g. water temp, DO, conductivity, pH and nitrate conc.) between downwelling and upwelling zones were shown along bar, indicating that hyporheic zones in gravel bars can functions as thermal and biochemical buffers. Based on the Trinity River results, we are creating a comprehensive picture of the physicochemical habitat complexity needed by salmon and macroinvertebrates, with a view toward management of quantity and quality of riverine organic matter in ecosystem recovery.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMEP23C0827O
- Keywords:
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- 0454 BIOGEOSCIENCES / Isotopic composition and chemistry;
- 0481 BIOGEOSCIENCES / Restoration;
- 1813 HYDROLOGY / Eco-hydrology;
- 1825 HYDROLOGY / Geomorphology: fluvial