Sequential riparian vegetation alteration in Japanese river landscapes

ABSTRACT

Bare gravel bed channels have been a common landscape feature of the middle reaches of rivers throughout Japan. They are disappearing these days because of vegetation colonization. Vegetation coverage of the riparian zones in the midstream reaches of rivers distributed throughout Japan was investigated using aerial photo images taken in 1946, 1975, and 2010. It was then compared with hydrological and water quality records, locations of dams and weirs, residential conditions of the surrounding areas, sediment yields, and forest coverage of the upstream mountainous areas. Riparian vegetation coverage has substantially increased in the last 70 years, from nearly 0 to 30–40%. Herbaceous vegetation coverage is higher than tree coverage. The total vegetation coverage did not correlate with water quality parameters. However, vegetation coverage had a significant negative correlation with the upstream basin's sediment yield and a slight negative correlation with the maximum flood peak flow rate. It was positively correlated with the terrestrial area fraction between levees. Riparian vegetation coverage was high within the 2 km downstream of dams or weirs, intercepting sediment flows. These results indicated that the gravelly sediment supplied from the upstream and deposited in the midstream at flood time has been the major contributor to keeping the vegetation coverage low rather than flushing by flood water. Reforestation since World War II has reduced the coarse sediment supply into the midstream river channels. This reforestation, together with gravel mining and intercepting sediment flow by dams and weirs, has caused the recent increase in vegetation coverage of the riparian zones. Human intervention can affect the vegetation conditions in riparian zones by decreasing sediment supply to river channels.

KEYWORDS:

• Riparian vegetation
• gravelly river channel
• sediment yield
• reforestation
• human intervention

Disclosure statement

No potential conflict of interest was reported by the author(s).

Author contributions

TA contributed to the conceptualization, filed works, and wrote the manuscript together with other members, DG reviewed, commented on the manuscript, and added Spanish examples, MO reviewed and commented on the manuscript, and MR edited throughout.

Data availability statement

The authors highly appreciate and state that data will be available for everyone upon reasonable request.

Additional information

Funding

This work was financially supported by the Grant-in-Aid for Scientific Research (B) [grant number 19H02245], (C) [grant number 20K04714], and the Fund for the Promotion of Joint International Research [grant number 18KK0116] of Japan Society for the Promotion of Science (JSPS).