Large wood supports hydrologically variable floodplain environments and aquatic biodiversity

It is increasingly recognized that riparian vegetation and wood play
crucial roles in enhancing spatial heterogeneity in rivers. Historical
removal of large wood and cutting of streamside forests around the world
have simplified rivers and substantially reduced aquatic habitat
complexity. To understand how (semi-)natural rivers sustain complex
geomorphology and hydrology, and how they support aquatic diversity, we
carried out an interdisciplinary field study in a stream flowing through a
natural forest in Hokkaido, Japan. We mapped 65 side channels (a total of
10.1 km long) along a 9.2 km length of the main channel. More avulsions
and side channels were present in stream sections with a higher density of
large wood and logjams. At flood stage, 90% of the side channels were
inundated with through-flowing river water, while the other 10% remain
disconnected from the river and harbor stagnant water. At low flow, only
20% of the side channels had flow from the mainstream river, 46% contained
stagnant water, and 34% were dry. In sum, the total surface area of side
channels with permanent flow was 20%, side channels with transient flow
were 9%, and disconnected side channels were 6% of the surface area of the
main channel. We estimated the contribution of each habitat type with
different hydrological regimes to the entire populations of aquatic
animals in the studied segment. The analysis demonstrated that four out of
eleven fish taxa, five out of 26 benthic macroinvertebrate taxa, three out
of three plankton taxa, and two out of two amphibian taxa primarily
utilized transient or disconnected channels. Furthermore, cohort analysis
of dominant fish Salvelinus leucomaenis showed that they exhibit
ontogenetic habitat shifts: they primarily utilize channels with transient
flow as juveniles and then shift their primary habitats to channels with
permanent flow, indicating the need for both types of habitats and their
connectivity. Our results demonstrate form and process characteristic of
the mostly lost and forgotten “baseline” of rivers in Japan; how
geomorphologically and hydrologically complex a natural river can be; and
how aquatic organisms rely on such complexity. Large wood potentially
plays important roles in sustaining such complexity, and further studies
should investigate these mechanisms.