2.1 Study system
The study was conducted on the Butokamabetsu River floodplain, located
in the Hokkaido University Uryu Experimental Forest, northern Japan
(44°24’N, 142°13’E) (Fig. 2). The floodplain is covered predominantly by
1–2 m high broad-leaf bamboo (Sasa senanensis ) and a sporadic
mix of deciduous trees such as alder (Alnus japonica ) and
Japanese white birch (Betula platyphylla ). Some wetlands and
areas of humic soils are distributed on the floodplain (Shibata et al.,
2004; Ogawa et al., 2006). The mainstem of the Butokamabetsu River,
which is about 10 m wide under summertime low-flow conditions, has many
extant and paleo-side channels. Extant side channels remain connected to
the mainstem even during periods of low flow and the water remains
flowing year-round. Paleo-side channels are old side channels that have
become isolated from the mainstem through the loss of their upstream
connection with the river because of fluvial sediment and wood
accumulation and flood dynamics. Many paleo-side channels, including our
study sites, harbour stagnant water during low flow (also called oxbow
lakes). Floodwater flows into some of the paleo-side channels at high
flow, and the water keeps flowing through these channels until the
mainstem water level drops to a level specific to each paleo-side
channel, depending on the side channel’s morphology. Therefore, a
gradient of hydrological connectivity to the river exists among
waterbodies on the floodplain: extant side channels, seasonally
connected paleo-side channels, and permanently isolated paleo-side
channels.
The study area receives mean annual precipitation of 1236 mm, largely as
snow, and snow cover usually lasts from late November to early May, with
a maximum depth of ~3 m (Aoyama et al., 2011). Snowmelt
typically occurs from April to May, and the river discharge is elevated
over this period (Fig. 2). Various aquatic animals including amphibians,
fishes, aquatic insects, and planktons take advantage of this yearly
snowmelt flooding for dispersal, spawning opportunity, and other
functions, and Uno et al. (2022) have previously documented that the
hydrological dynamics over the snowmelt period shape the spatially
heterogeneous aquatic communities in the floodplain. Besides snowmelt
flooding, the area also experiences summer spates due to rain, which can
cause higher peak river discharge but are much shorter in duration than
the snowmelt flood and usually last only for a few days.