Evaluation of Cross-sectional Geomorphology for Lentic Habitat Restoration in the Uji River

Abstract

Most of rivers in civilized countries have lost biodiversity due to deterioration of habitats by the artificial river works such as channelization, sand mining and dam construction. The Uji River is a typical river suffered by degradation and bipolazation of the riverbed due to reduced sediment dynamism and step-wise flow regime caused by the gate operation at Seta Barrage for control the discharge from the Lake Biwa, leading to decreased the connectivity of the main channel and the bank side lentic habitats. In this paper, we analyzed relations of aquatic animal communities in lentic habitats to the riverbed geomorphological parameters such as relative height of the habitat to the main channel, the length of cross-sectional perimeter, floodplain vertical shape index (FVSI), and clay layer distribution. Based on the results, the lentic habitats were classified into three types: 1) Low relative height ponds with high biodiversity with taxonomic groups common with lotic habitats, 2) terrace ponds with intermediate and high relative height with low biodiversity with taxonomic groups common with moor-like still water habitats, 3) island bar ponds with high relative height with rather high biodiversity. In addition, we found that the location of these lentic habitats showed a fairly good correspondence to the clay layer distribution. Based on these interrelationships, we proposed a method for evaluation of lentic habitat potential combining a suitable relative height with the underground clay layer distribution. This method will be applicable to nature restoration plans for creation of lentic habitats in degraded rivers in general as well as in the Uji River.