Historical removal of large wood and cutting of streamside forests around the world have simplified rivers and substantially reduced aquatic habitat complexity. In this talk, I will briefly introduce two interdisciplinary field studies in Montana, USA and Hokkaido Japan to demonstrate how natural rivers sustain complex geomorphology and hydrology, and how they support aquatic diversity. Both rivers sustain pristine multi-thread river channels flowing through natural forests with large woods, and spatial analysis in both rivers have shown presence of such side-channels are associated with logjams in the river corridors.
Historical aerial imageries of the Swan River in Montana showed us how such river channels have dynamically moved around on the floodplain over decades. To test a hypothesis that such dynamic movements of channels form spatially variable habitats in a river and support diverse biota, I have conducted field surveys of thirty side channels with different histories (widening, stable or narrowing). At each channel, I measured physical environment, and surveyed various aquatic and riparian biota (benthic macroinvertebrates, fish, amphibians, birds). Newly forming widening channels tended to be steeper, deeper and faster flow, and had more canopy cover, and there were higher density of large fish. In contrast, narrowing channels tended to be shallower, lower flow, and had higher canopy cover, and there were higher density of small fishes and frogs. Spring channels formed as legacy of the side channels also supported unique habitat with cold water and stable habitat, and higher density of benthic macroinvertebrates were found.  Path analysis integrate the processes how the presence of logjams support channel dynamics, diverse physical environment, and hence diverse biota in a natural floodplain.
In Butokamabetsu river, in Uryu research forest in Hokkaido, Japan, we have been conducting multi-year field-based research to associate the river dynamics, physical and chemical environment and habitat use by aquatic biota. The research shows how flow dynamics and change in geomorphology of the channels shape the floodplain ecosystem each year.