Investigation into the Impact of Morphologic and Hydraulic Characteristics on In-channel Vegetation in the Tamagawa River

Abstract

The impact of channelization and flow regulation by dams and weirs on river habitats and landscape has been enormous. The most serious impact of river works in Japan, in order to meet the needs of flood protection and water resources, has been to lose a great variety of riverine plants. This has resulted in habitat loss for wildlife and reduced the ability of the river to carry out many of its natural functions, that is the control of local scouring in a levee, water purification and sediment storage. However, only few attempts have so far been made at the evaluation about progression characteristics of vegetation in channelized river and there is a necessity to consider of the potential opportunities for habitat enhancement and rehabilitation which may be undertaken during engineering works. This research attempts to evaluate the progression characteristics of in-channel vegetation in Tamagawa River by aerial photographs. The research has also shown the relationships between the progression characteristics of in-channel vegetation and the impact of the morphologic and hydraulic characteristics included by human impact on in-channel vegetation.

Spatial–temporal distribution of ornithochorous seeds from an Elaeagnus umbellata community dominating a riparian habitat

“…For example, the persistence of Salicaceae trees alters the riverbed microtopography by entrapping sandy sediment (Kamada et al . 1997), and has created high mounds towards the downstream area, providing establishment sites for other tree species including invasive alien species (Lee et al . 1996; Kamada 2004; Muranaka 2009).…”

“…For most riparian tree species that are expanding their distribution, dam construction has coincidently facilitated seedling survival and resulted in an unusual expansion and succession of riparian woodland in many parts of the world (Hupp 1992;Johnson 1994;Nakamura et al 1997;Friedman et al 1998). For example, the persistence of Salicaceae trees alters the riverbed microtopography by entrapping sandy sediment (Kamada et al 1997), and has created high mounds towards the downstream area, providing establishment sites for other tree species including invasive alien species (Lee et al 1996;Kamada 2004;Muranaka 2009). As vegetation develops, the extensive biomass regulates a smooth river flow discharge (Darby 1999), but the presence of embankments restricts the movement of gravel bars (sensu Grunell 1995; Sakio 1997), and ultimately tends to limit new regeneration sites for future E. umbellata communities.…”

Expansion of Elaeagnus umbellata on a gravel bar in the Naka River, Shikoku, Japan

“…It has also become apparent in Japanese floodplains over the past few decades. For example, there was an obvious increase in woody vegetation on gravel bars in the Tama River, Tokyo Prefecture, Japan, during the 1980s (Lee et al . 1996).…”