Why did henando de soto's men burry him in the Mississippi river?

Answer 1

DeSoto had told the local Natives that he was a god in order to make them wary and respectful, and keep the expedition safe. When he died of fever, his men at first buried him in the ground and various writings say the Natives noticed the disturbed soil - and presumably DeSoto's absence. Fearing the Natives would dig him up and see that the "god" was just, in fact, a mortal, his men dug him up and deposited his weighted corpse in the waters, telling the Natives afterward that DeSoto had returned to the Sun.

From Wikipedia:

De Soto died in 1542 on the banks of the Mississippi River at present-day Lake Village, Arkansas. Lake Village is named for its location on Lake Chicot, an oxbow lake formed from the Mississippi River. According to legend, the remains of DeSoto are buried in Lake Chicot (the lake is approximately 3/4 mile wide and 22 miles long. See... http://en.wikipedia.org/wiki/Lake_Chicot )

Answer 2

They had told the local population that Hernando was an immortal Sun God, so he couldn't very well show up dead. So they sunk him in the Mississippi (or, according to others, in Lake Chicot) in the middle of the night.