Coupled effects of dam, hydrology, and estuarine filtering on dissolved organic carbon and optical properties in the reservoir-river-estuary continuum

•DOC and S275-295 increased but a280 and SUVA254 decreased in the surface reservoir.•DOC and optical indices were stable in the river zone in the normal and dry seasons.•DOC and a280 in the river zone showed evident additions during the rainy season.•CDOM and humic-like C1 and C3 correlated strongly with salinity in the estuary.•The protein-like C2 and C4 did not correlate with salinity in the estuarine zone.