 On the night of Operation Chastise, 16 / 17 May 1943, Shannon was among a group of Lancasters led by Gibson in assaults on the Möhne and Eder Dams. After five of the aircraft had dropped their bouncing bombs on the Möhne, Shannon was preparing to make his attack on the dam when it gave way, so he carried on to the Eder with Gibson and three other Lancasters, captained by "Dinghy" Young, Henry Maudsley, and Les Knight. Detailed for the first bombing run at the Eder, Shannon took several attempts to familiarise himself with the area and line up his aircraft, so in the meantime Gibson ordered Maudsley to make his attack. Shannon went in next, delivering his bomb on target. Knight then dropped his bomb, and the dam broke. Shannon landed back at RAF Scampton feeling "terribly elated". His bomb was believed to have caused a crack in the dam's wall, while Knight's completed the breach. Speaking later of the severe losses suffered by the squadron — eight out of nineteen planes — Shannon contended, "I suppose we had become hardened to loss – we could shrug it off. We had to, otherwise we could never have flown again." He was awarded the Distinguished Service Order for his part in the mission, one of a number of decorations awarded to the aircrew of No. 617 Squadron for the dams raid and promulgated in the London Gazette under the citation:
