PARTICULATE ORGANIC MATTER RETENTION AS AN
ECOLOGICAL INDICATOR FOR RIVERBED MANAGEMENTGIYOUNG OCK1,2 YASUHIRO TAKEMON1 AND TETSUYA SUMI1
1Water resources research center, Disaster Prevention Research Institute,Kyoto University, Uji, Kyoto, 611-0011, Japan.
Institute of urban and regional development, University of California at Berkeley,
300 Wurst hall, Berkeley, CA 94706, USA (Current).
(Tel: +81-774-38-4317, Fax: +81-774-38-4036, e-mail: ock@ecohyd.dpri.kyoto-u.ac.jp)

2Reservoir dams are known to degrade ecological functions of riverbed in dam
downstream reaches by alteration of channel geomorphology (e.g., riverbed degradationand channel incision) caused by reduced bed load transport. Recent river management
particularly in dam tail waters has a growing concern on riverbed management (i.e.,through sediment replenishment, bypass tunnel, dry dam without impoundment) forrecovering functions of habitat conditioning and of material cycling acceleration. From aperspective of riverbed management considering sediment and flow regime controls,
therefore, development of ecological evaluation methodology for desirable
geomorphology is increasingly required (Takemon 2011). Retention of particulate organic
matter (POM) is an important capacity of riverbed, because it retains suspended materials
leading to provide energy resources to heterotrophic foodwebs as well as to enhance
purification. In particular, dam downstream ecosystems disturbed by input of large
amount of plankton from reservoir outflows are supposed to be highly reliant on theplankton as major trophic source. Therefore finding an effective riverbed conditions forincreasing the retention of lentic plankton is critical to restore the degraded ‘reservoirriver hybrids’ to a normal state of ‘lotic ecosystem’ (Ock and Takemon 2010).
In the present study, we investigated the downstream changes of POM sourcecomposition and concentrations aiming to evaluate roles of (1) riffle-pool reach in two
dam tailwaters in Japan (Uji R. changed to short riffles-long pools by riverbed
degradation and Nunome R. maintaining long riffles-short pools by sediment
replenishment) and (2) gravel bar in the braided reaches of Tagliamento River in Italy,
characterized by highly dynamic fluvial geomorphology created by natural flooding
events and sediment supply). Advanced analyses of stable isotopes (δ15N and δ13C) andC:N ratio can identified the three sources effectively, and estimate their relative
contributions longitudinally. The results showed that the relative contribution of lentic
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plankton trended to be reduced steeply during passing riffles, whereas that of terrestrial
plant decreased during passing pools, indicating that riffle-pool geomorphology havesorting capacity to POM components according to its source origins. On the other hands,
the braided channel in Tagliamento River showed high POM retention capacity whencompared with other deep channels. Moreover, the hydromorphological heterogeneity
within an island bar resulted in source-specific retention process and capacity along
shoreline (Figure 1). Allochthonous and autochthonous particles were highly depositedalong barhead and bartail shorelines, respectively. This spatial diversity of POM quantityand quality along shore will lead to highly heterogeneous habitats in braided channels.
Overall, the results suggest the significance of riffle-pool and bar geomorphology inriverine POM retention for enhancing trophic spiraling efficiency of nutrients and forproviding spatial heterogeneity of trophic origin of POM. They can contribute toevaluation of riverbed management in consideration of ecological functions.

Figure. 1 Spatial distribution of standing stock of benthic POM (lower) and results of
their source composition analysis by stable carbon isotope and C/N ratios(upper) in a braided island bar in Tagliamento River. The bar graph and arrowmeans the density of benthic POM, and secondary flow velocity and direction,respectively.
1 Ock G. and Takemon Y. (2010), “Estimation of transport distance of fine organicmatter in relation to channel morphology in tailwaters of the Lake Biwa andreservoir dams” Landscape and ecological engineering, Vol 6, 161-169.
2 Takemon Y. (2010), “Habitatology for linking sediment dynamism and ecology”,
proceeding of International symposium on sediment disaster and river environment
in mountainous area, Kyoto, Japan, 25-32.

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