While we were in the sandhills, scouting the Niobrara country, the Pawnee Indians brought into camp some very large bones, one of which the surgeon of the expedition pronounced to be the thigh bone of a human being. The Indians said the bones were those of a race of people who long ago had lived in that country. They said these people were three times the size of a man of the present day, that they were so swift and strong that they could run by the side of a buffalo, and, taking the animal in one arm, could tear off a leg and eat it as they ran.
These giants, said the Indians, denied the existence of a Great Spirit. When they heard the thunder or saw the lightning, they laughed and declared that they were greater than either. This so displeased the Great Spirit that he caused a deluge. The water rose higher and higher till it drove these proud giants from the low grounds to the hills and thence to the mountains. At last even the mountaintops were submerged and the mammoth men were drowned.
After the flood subsided, the Great Spirit came to the conclusion that he had made men too large and powerful. He therefore corrected his mistake by creating a race of the size and strength of the men of the present day. This is the reason, the Indians told us, that the man of modern times is small and not like the giants of old. The story has been handed down among the Pawnees for generations, but what is its origin no man can say.
http://www.wordowner.com/cody/chapter6.htm
Sound like the Geber, the offspring of the Nephillim — the Fallen Angels— & the daughters of man, against whom God sent the flood of Noah.