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.