Flushing sediment from reservoirs triggers forestation in the downstream reaches

Abstract

It was assumed that sediment release from a reservoir would trigger forestation and change the spatial pattern of tree recruitment in the downstream. This study tested these hypotheses by observation on a downstream sandbar of the Kurobe River. Prior to sediment release, Elaeagnus umbellata and Salix gilgiana were the most abundant tree species (33% and 64% of the total tree population) on the bar. The former colonized on elevated areas, whereas the latter mostly grew along the edges of river channels. After 2008 and 2009 sediment releases, a thick layer of fine sediment accumulated in active river channels and on low‐elevated areas of the bar. The particle size of the accumulated sediment was significantly lower (p < 0·01) and the moisture content was higher (p < 0·01) than in the underlying substrate. Total nitrogen, carbon and phosphorus concentrations of the accumulated sediment were higher than in the covered portions. After sediment release, a seedling flushing of both species occurred on the bar where 87% was composed of S. gilgiana and the remaining 13% was of E. umbellata. Although most of the S. gilgiana seedlings recruited on low‐lying areas, numerous individuals also grew on elevated margins of the channels and on areas dominated by E. umbellata. The accumulated sediment made changes in the substrate, which were favourable for the recruitment and spread of S. gilgiana. This study suggests that an alternative to sediment flushing for reservoir maintenance should be sought or the impacts on downstream environment should carefully be considered before an extensive sediment flushing operation. Copyright © 2014 John Wiley & Sons, Ltd.

The effect of the shortage of gravel sediment in midstream river channels on riparian vegetation cover

“…Figure shows the observed transition of the vegetation cover on a depositional site of the bar in 2007 located 4.4 km upstream from the river mouth after a flood in 2007 in the Kurobe River. Since the start of sediment release, the particle sizes have changed to predominantly sand and silt at the depositional site (Asaeda, Rashid, and Sanjaya, ).…”

“…Moreover, the influence of the soil particle size and nutrient retention in riparian soil is well documented (Bechtold & Naiman, ; Pinay, Ruffinoni, & Fabre, ). Asaeda, Rashid, and Sanjaya () reported a negative relationship between the soil TN content and particle size in riparian soil with increased particle size. Therefore, gravelly substrate lacks fine sediment and organic matter, which retain moisture and nutrients for a long period after a flood.…”

“…Many native herbaceous species have disappeared, and invasive woody species, such as Robinia pseudoacacia , have rapidly expanded, reducing the flood‐routing capacity (Kuramoto, Takenaka, Washitani, & Inoue, ; Miyawaki & Washitani, ; Nakamura, Tockner, & Amano, ). Many studies have investigated the recent extensive afforestation of riparian zones (Asaeda, Rashid, & Sanjaya, ; Kurokochi & Hogetsu, ; Nakamura et al, ; Yoshimura, Omura, Furumai, & Tockner, ), but the exact reason remains unclear. In the meantime, past extensive river gravel mining, intensive river regulation activities, and afforestation in river basins, which all limit sediment production, have occurred in almost all the rivers in Japan (Asaeda & Sanjaya, ).…”

The Palimpsest of River-Floodplain Management and the Role of Geomorphology

“…For the purpose of reducing the downstream impact of dams, a measure gaining in use is the downstream flushing of sediment trapped within reservoirs. The procedure is complex to implement, depending largely upon the configuration and morphology of the river valley, sediment type, as well as the dam and reservoir operation and design (Asaeda et al 2014). Experiences along the Rhône River, for example, have been somewhat effective at reducing the impact of hungry water.…”

Eco‐hydraulic evaluation of herbaceous ecosystems below headwater dams without a base flow: Observing below dam reaches as new stream sources

“…However, there are also reports on enhanced downstream vegetation (Azami, Suzuki, & Toki, 2004;Asaeda, Gomes, & Takeda, 2010). The stabilization of downstream soil bars is known to foster the early development of vegetation, generally starting with herbaceous plants (Asaeda, Rashid, & Sanjaya, 2015;Sun, Deng, Yuan, Liu, & Zhang, 2014;Gomes & Asaeda, 2009).…”