The mission grew out of a concept for a bomb designed by Barnes Wallis and developed by his team at Vickers. Wallis was an aircraft designer who had worked on both the Vickers Wellesley and Vickers Wellington bombers. While working on aircraft he had also begun work on a bomb designed specifically with dam-destruction in mind.
Wallis' concept was a drum-shaped bomb spinning backwards at over 500 rpm, dropped at a sufficiently low altitude at the correct speed. It would skip for a significant distance over the surface of the water in a series of bounces before reaching the dam wall. Its residual spin would run the bomb down the side of the dam to its underwater base. Using a hydrostatic fuse, an accurate drop could bypass the dam's defences and enable the bomb to explode against the dam.
After some testing and many engineering meetings, the idea was adopted on 26 February 1943. The bomb was codenamed "Upkeep", and a target date was set for May, when water levels would be at their highest and breaches in the dams would cause the most damage.
At least 1,650 people were killed: around 70 in the Eder Valley, and at least 1,579 bodies were found along the Möhne and Ruhr rivers, with hundreds missing. 1,026 of the bodies found downriver of the Möhne Dam were foreign prisoners of war and forced-labourers in different camps, mainly from the Soviet Union. Worst hit was the city of Neheim (now part of Neheim-Hüsten) at the confluence of the Möhne and Ruhr rivers, where over 800 people perished, among them at least 526 female forced-labourers from the Soviet Union. (Some non-German sources erroneously cite an earlier total of 749 for all foreigners in all camps in the Möhne and Ruhr valleys as the casualty count at a camp just below the Eder Dam.)
After the operation Barnes Wallis wrote, "I feel a blow has been struck at Germany from which she cannot recover for several years." However, on closer inspection, Operation Chastise did not have the military effect that was at the time believed. By 27 June full water output was restored, thanks to an emergency pumping scheme inaugurated only the previous year, and the electricity grid was again producing power at full capacity. The raid proved to be costly in lives (more than half the lives lost belonging to Allied POWs and forced-labourers), but in fact no more than a minor inconvenience to the Ruhr's industrial output. The value of the bombing can perhaps best be seen as a very real boost to British morale.
By far the greatest and most unexpected effect was on German food production. The Ruhr Valley below the dams was a major source of vital food for Germany, and large areas of arable land were rendered unusable and huge numbers of farm animals were killed. This had an immediate negative effect on German morale. In addition, the pictures of the broken dams proved to be a propaganda and morale boost to the Allies, especially to the British, still suffering under German bombing.