Can an alternate biogeomorphological state be triggered by short-term
hydrological fluctuations in modified monsoon-driven river systems?
Abstract
Over the last decade, rapid vegetation colonization and changes in
channel morphology have been observed in the Naeseongcheon Stream in
South Korea, which were linked to short-term hydrological fluctuations
under a changing monsoon climate. The surface area covered by vegetation
has been increasing; this increase intensified after the 2014–2015
drought, which provided a window of opportunity for vegetation
establishment. During the drought, pioneer herbaceous vegetation densely
colonized the lower floodplains, including bare sandbars and temporarily
exposed riverbed. Although the colonized lower floodplain and river
banks were partially rejuvenated by several subsequent floods,
succession to woody vegetation continued, resulting in stable vegetation
cover in areas that had previously been bare. Moreover, sediment carried
by flood water was deposited on and around the vegetated areas, and the
low-water channel was incised, causing vertical development of river
topography. In addition, the main channel width decreased in previously
relatively wide sites, and secondary channels formed. The results of
this study show that river rejuvenation by floods may decrease owing to
systemic changes in the river system. Therefore, we concluded that the
Naeseongcheon watershed was primed by human-induced changes, which made
the river system more susceptible to changes in rainfall and discharge
due to climate change. Furthermore, after the initial vegetation
colonization, changes in nutrients and temperature created a positive
feedback loop, which reinforced vegetation establishment.