The largest flood in modern recorded history, in terms of discharge, occurred in 1841 on the Indus River. It was caused by a landslide that had blocked the river, resulting in a natural dam. The water behind the dam mounted until the dam failed, causing a torrent which rushed through at an estimated 540,000 cubic meters per second.
Anything downstream of this deluge would have been completely washed away. In fact, a Sikh army, fighting in the Sino-Sikh War, was reportedly drowned by these waters as far as 300 miles downstream from the original breach.