Long-term riverbed response of lower Tedori River, Japan, to sediment extraction and dam construction

207

113

Sand mining (used here as a generic term that includes mining of any riverine aggregates regardless of particle size) is a global activity that is receiving increasing media attention due to perceived negative environmental and social impacts. As calls grow for stronger regulation of mining, there is a need to understand the scientific evidence to support effective management. This paper summarizes the results of a structured literature review addressing the question, “What evidence is there of impacts of sand mining on ecosystem structure, process, and biodiversity in rivers, floodplains, and estuaries?” The review found that most investigations have focused on temperate rivers where sand mining occurred historically but has now ceased. Channel incision was the most common physical impact identified; other physical responses, including habitat disturbance, alteration of riparian zones, and changes to downstream sediment transport, were highly variable and dependant on river characteristics. Ecosystem attributes affected included macroinvertebrate drift, fish movements, species abundance and community structures, and food web dynamics. Studies often inferred impacts on populations, but supporting data were scarce. Limited evidence suggests that rivers can sustain extraction if volumes are within the natural sediment load variability. Significantly, the countries and rivers for which there is science‐based evidence related to sand mining are not those where extensive sand mining is currently reported. The lack of scientific and systematic studies of sand mining in these countries prevents accurate quantification of mined volumes or the type, extent, and magnitude of any impacts. Additional research into how sand mining is affecting ecosystem services, impacting biodiversity and particularly threatened species, and how mining impacts interact with other activities or threats is urgently required.

Section: Abiotic Impacts To River Systemsmentioning

confidence: 99%

Impacts of riverine sand mining on freshwater ecosystems: A review of the scientific evidence and guidance for future research

Koehnken¹,

Rintoul

Goichot³

et al. 2020

207

113

“…Thus, as one of the steepest river basins in Japan, more than 90% (743 km 2 out of 809 km 2 ) of the Tedori River basin is in mountainous areas, and consequently, this is a river system with a high flow energy. As a result, the mean sediment diameter of the riverbed varied from 40 to 440 mm, as surveyed in 1993 (Dang et al, 2014). The river reach used in the study was the downstream-most reach of the lower Tedori River.…”

Section: Study Areamentioning

confidence: 99%

“…Since Dang et al (2014) reported decadal variations in the sediment budget components of the lower Tedori River, the relative change in the extent of deposition and erosion reflected by in-and-out sediment conveyance values can be clearly seen in the response to flow stabilization due to dam operations after 1979. In the lower 16 km river reach, a more than twofold contraction of sediment inflow occurred after 1979 in terms of volume (from 0.28 to 0.11 × 10 6 m 3 per year) was observed.…”

Section: Remodelling Of the Riparian Ecosystemmentioning

confidence: 99%

The long‐term legacy of riparian vegetation in a hydrogeomorphologically remodelled fluvial setting

Nallaperuma

Asaeda

2020

The typical Japanese river maintains a vegetation-suppressive effect on riparian habitats. During recent decades, most Japanese rivers have encountered extensive human-induced alterations, and over time, the rivers have been transformed from unvegetated to vegetated. The lower Tedori River, which is a representative case of this phenomenon, is located in a region that exhibits signs of global climate change. According to quantitative analyses of sediment, the river corridor has historically reached an equilibrium since the prohibition of heavy sediment extraction activities, and the water discharge is stable because of dams. The contemporaneous vegetation encroachment was observed in a historical imagery survey and the vegetation dynamics of the river reach for the past 18 years were analysed to identify forcing hydro-climatic variables. The normalized difference vegetation index (NDVI) was adopted for the surveillance of vegetation dynamics, and multiple regression analysis was employed to evaluate its relationship with predictor variables. River water level was the strongest determinant of NDVI, with both Pearson correlation and standardized β coefficients of −0.405, while air temperature was next, with values of 0.363 and 0.288, respectively. These findings were supported by the spatial distribution of temporally advancing vegetation patches determined using the aerial imagery and pixel value maxima of NDVI bands. In addition, the development of vegetation patches will presumably give rise to more pro-vegetation surroundings in the riparian zone through reciprocal linkages with hydrogeomorphic processes. These objective predictions may help inform the proactive planning for river and coastal management.

“…Overall, the riverbed became coarser (Table ). Coarsening of the riverbed is associated with riverbed erosion, but when erosion exposes the bedrock or the flow velocity is not sufficient to remove sandy gravel, erosive incision ceases or slows (Assani & Petit, ; Dang et al, ). Riverbed coarsening clearly occurred in downstream channels near the dam, whereas channels with riverbeds characterized by sandy gravel increased farther downstream.…”

Section: Effects Of Water‐sediment Conditions and Human Activities Onmentioning

confidence: 99%

“…In the past few decades, extreme natural events and human activities have had a significant influence on riverbed deformation (Dang, Umeda, & Yuhi, 2014;Darby et al, 2016;Mei et al, 2018;Rinaldi & Simon, 1998). Human disruptions can rapidly accelerate or change natural processes and trends, thereby shortening the time scale for river channel adjustments (Rinaldi & Simon, 1998).…”

Section: Introductionmentioning

confidence: 99%

Impact of the operation of a large‐scale reservoir on downstream river channel geomorphic adjustments: A case study of the Three Gorges

Yang

Zhang

Zhu

et al. 2018

To evaluate the effects of reservoir construction on the geomorphology of downstream reaches, a case study is performed on the Three Gorges Reservoir (TGR) by comparing data collected before and after impoundment of the reservoir. This study focuses on a 1,183‐km‐long reach downstream from the TGR and investigates the temporal and spatial characteristics and patterns of geomorphic adjustment in the downstream reaches. The amount of erosion in flood plain channels and the annual average intensity of erosion in downstream reaches after 12 years of impoundment are larger than the predicted values before the impoundment of the TGR. The maximum intensity of erosion was found at 410 km from the dam, with an average thalweg incision of 1.5 m, and the main zone of erosion migrated approximately 80 km downstream. At both preimpoundment and postimpoundment, erosion occurred the along beaches and channels in the Yichang–Chenglingji reach. The pattern of erosional channels and depositional river floodplain changed to erosional channels and erosional river floodplain in the Chenglingji–Hankou reach after the trial impoundment stage. In the Hankou–Hukou reach, the erosional channels and depositional river floodplain pattern changed to erosional deep channels, erosional low beaches, and depositional high‐river floodplain during the early impoundment stage. After the trial impoundment stage, erosion continued in the deep channels, but minor deposition occurred on the river floodplain. Moreover, the discharge of clear water increased, and the duration from middle to low‐water levels increased, leading to more concentrated erosion in low‐water channels. The intensity of the erosion along the river floodplain decreased due to the shortened duration of high‐water levels and reduced human activity.