Carbon dioxide degassing and lateral dissolved carbon export during the unprecedented 2019 Mississippi river mega flood – Implications for large river carbon transport under future climate

•The 2019 mega Mississippi River flood transported 3.73 Tg C DOC and 14.14 Tg C DIC.•Average CO2 degassing rate of 3.67 g C m−2 d−1 during the extreme flood was very high.•Increased floodplain connectivity and temperature affected dissolved carbon transport.•Exclusion of floods can lead to large uncertainties in river carbon budget estimates.