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

摘要

The roles of headwater streams are usually underestimated and inadequately managed compared with the larger downstream river systems. This two and half-year study covering several wet and dry seasons evaluated the impacts of small headwater dams without a base flow on downstream herbaceous dynamics. Results showed herb aerial cover and diversity below dams (new stream sources) are unique and different to original stream sources as well as to a comparable unregulated reach. The geomorphological habitats formed subsequent to damming did not show major influence on aerial cover of herbs; nevertheless, diversity showed a clear dependence. Downstream of dams were dominated by Acorus gramineus, which had >40% aerial cover, and this figure increased up to 60% within 2years (in between a major flood event). Simultaneously, downstream herb diversity decreased by 33%. However, diversity and aerial cover of the unregulated reach and original sources remained more or less the same. Herb pulling tests (simulating an uprooting mechanism during a flood) showed A.gramineus can withstand an eightfold higher force than the other herbs, especially in concrete surfaces and rock/sand pools. The estimated shear force on bed during the major flood was several folds less than the observed. This suggests downstream of dams behave opposite to the conventional understanding that large floods are considered as an agent of restoration.