Guan Yu comes up with a plan to flood the River Xiang to wipe out the Wei army

Looking round, Guan Yu noted that there seemed much slackness about everything---from flags to soldiers---in Fancheng, that the relief armies were camped in a valley to the north, and that River Xiang seemed to run very swiftly. After impressing the topography on his mind, he called the guides and asked the name of the gully about three miles north of the city.

"Zengkou Stream," was the reply.

He chuckled.

"I shall capture Yu Jin," said he.

Those with him asked how he knew that.

He replied, "Why, how can any fish last long in such a trap?"

Those in his train gave but little weight to what he said, and presently he went back to his own tent. It was just then the time for the autumn rains, and a heavy downpour came on, lasting several days. Orders were given to get ready boats and rafts and such things. Guan Ping could not think what such preparations meant in a dry land campaign. So he asked his father.

"Do you not know even?" replied his father. "Our enemies have camped in difficult ground instead of the open country and are crowded in the dangerous valley there. After some days of this rain, River Xiang will swell, and I shall send people to dam up all the outlets and so let the water rise very high. When at its highest, I shall open the dams and let the water out over Fancheng. That valley will be flooded too, and all the soldiers will become aquatic animals."

...

In this flood perished the whole of the seven armies, except the few that saved themselves by swimming. These latter, having no way of escape, surrendered to the victors.

In the depth of night rolled the war drums,
Summoning the warriors as to battle;
But the enemy was no man,
For the waters had risen and the flood came.
This was the plan of Guan Yu, the crafty,
To drown his enemies. More than human
was he in cunning. The ages hand on his fame
As his glory was told in his own day (Romance of the Three Kingdoms, Ch 74).