3.4. Calculation of river organic carbon (OC) fluxes
In order to determine the particulate OC fluxes from rivers, we use bedload sediment discharge data published in past literature (Syvitski and Farrow, 1983, Text S2), recent total suspended sediment measurements conducted on the Homathko and Southgate Rivers (Text S3), and TOC measurements of river sediments collected in October 2017 (Table 1).
Bedload discharge for the Homathko River was estimated at 3100 Kt/yr by Syvitski and Farrow (1983). We scaled this Homathko bedload estimate to the Southgate River using the ratio of watershed areas (Table 1; Text S2). The yearly mean suspended load discharge for the Homathko and Southgate Rivers was estimated at 946 Kt/yr and 220 Kt/yr, respectively. This is based on suspended samples collected approximately monthly between 2018 and 2021 for the Homathko River (Text S3), together with daily water discharge data (Runkel et al. 2004; Text S3). There was no water discharge data available for the Southgate River up to 2021. Therefore, a linear regression was used to estimate daily Southgate River water discharge from Homathko River discharge, similar to Hood et al. (2020). The training dataset contained 123 daily flow observations spanning a broad range of flows from record high discharge in late June 2021 through moderately low flows in October 2021 (Text S1).
Finally, we derived both river OC fluxes by multiplying the average TOC between river bedload and suspended load samples with the annual bedload and suspended load discharge, respectively (Table 1).