2.1. Homathko and Southgate Rivers
The Homathko and Southgate Rivers feed the head of Bute Inlet (Fig. 1). Both rivers are supplied by meltwater from snow and glaciers within their catchment and by moderate to high levels of precipitation from a rainforest climate in the coastal mountain catchments (1684 and 2089 mm of mean annual precipitation were recorded between 1981 and 2010 in the Homathko and Southgate Rivers, respectively; Giesbrecht et al., in press). The river floodplains are populated by coniferous and deciduous forests. These two rivers provide 94 % of the freshwater and sediment inputs into the fjord, with the remainder of the freshwater being provided by small streams entering the fjord at its margins (Farrow et al., 1983). The watershed areas are 5782 km2 and 1985 km2 for Homathko and Southgate Rivers, respectively (Fig. 1; Gonzalez et al., 2018). The Homathko River has an average annual discharge of 250 m3/s, with an average summer peak discharge of ~800 m3/s, in response to melting snow and ice (Water Survey of Canada, 2020). The average annual suspended sediment load of the Homathko River is 30 kg/s, whereas its bedload is 99 kg/s (Table 1; Syvistki and Farrow, 1983). There was no gauging station in the Southgate River up to 2021. However, recent (2021) data and the co-variation between daily discharge on the two systems allowed the Southgate discharge (water and total suspended load) to be modelled for our years of interest (Table 1; Texts S1 to S4).
During the spring thaw (freshet), the rivers create large sediment-laden freshwater plumes at the fjord’s head (Tabata and Pickard, 1957, Syvitski et al., 1985). These plumes flow on top of the more saline fjord water, thereby bringing terrestrial particles into the fjord and enhancing the heterotrophic activity of bacterioplankton (Albright, 1983). Due to the relatively large tidal range (up to 5.5 m), sediment from these river plumes is likely to be concentrated in a turbidity maximum that is located in between the fresh and saline water (Dyer, 1997). Observations in a similar and nearby fjord setting (Howe Sound, Canada), show that during the freshet these turbidity maxima can hold enough sediment to become denser than the saline fjord water (Hage et al., 2019). During spring tide, the river plume becomes more focused and powerful, so that low tides cause these dense turbidity maxima to migrate onto the steep delta-slope, where they can trigger turbidity currents (Hage et al., 2019). These processes, together with submarine failures of the delta slopes, result in turbidity currents frequently being triggered in such river-fed and tide-influenced fjord settings (Clare et al., 2016; Hizzett et al., 2018). It is likely that similar initiating mechanisms for turbidity currents also occur in Bute Inlet (Hughes Clarke et al., 2015).