A Scenario of Area Expansion of Stable Vegetation in a Gravel-Bed River Based on the Upper Tama River Case

Abstract

Vegetation influence is not only important for the channel management, but also for natural habitats . Clearly, the expansion of in-stream vegetation found on the gravel bed depend on site factors, such as bed material, groundwater level and geology. Above other things, fine sediments play a important role. During floods, the deposit of fine sediments is affected by the numbers and types of vegetation, alterations of the volume and timing of flows, and changes in the amount and type of sediment through which the river with the gravel bed flows.The result of this research shows significant correlation in hydraulics between in-stream vegetation and the deposit of the fine sediment, and suggests the scenario of vegetation expansion in alluvial rivers.

River and Wetland Restoration: Lessons from Japan

“…During the past few decades, the river has evolved from a large, braided gravel-bed river to a singlethread, incised channel fringed by densely forested terraces (figure 3). This geomorphic transformation was caused by (a) gravel excavation during the rapid economic growth period in the 1950s and 1960s; (b) a decline in sediment supply from upstream sections; (c) flow control by the Hamura weir, located 0.5 km upstream; and (d) fine sediment deposition along the incised channel (Lee et al 1999, Minagawa and Shimatani 1999, Shimatani 2003. As a result, native plants and animals typical of bare sediments (e.g., Aster kantoensis) disappeared almost completely, while nonnative woods (e.g., Robinia pseudoacacia) rapidly expanded (Kuramoto et al 1992).…”

Nitrogen stocks in a riparian area invaded by N-fixing black locust (Robinia pseudoacacia L.)

“…4), but the N accumulation rates exceeded the N supply rates by black locust leaves. In addition to N fixation, the sediment N accumulation might be accelerated by sediment trapping effect due to riparian forest combined with undergrowth herbaceous plants (Hickin 1984;Lee et al 1999). The increase of N accumulation rate with distance from the river might result from combination of both N input by black locust (Fig.…”

“…Levee crevasses, river break structures such as bridge piers, and accumulated driftwood can cause an increase in water levels leading to flooding (Melville and Dongol 1992;Tanaka and Yagisawa 2009). These problems have potential to be accelerated by mobilization of fine sediment (Asaeda et al 2009), where black locust colonized (Maekawa and Nakagoshi 1997;Lee et al 1999;Fukuda et al 2005). River regulation has generally protected human lives and assets, and has helped sustain agroindustrial developments in river basins (Yoshimura et al 2005;Osugi et al 2007).…”