Nelson was in the water with his younger brother and cousin and was able to help get the other two boys to safety before going under.
The weather may have played a role in the accident. A storm blew through Madison about 2:00 Saturday afternoon.
Nelson, his younger brother, Andrew, and their cousin waited it out. Then, officials say, they went into the water, walking across the sandbar.
"The water was a little wavy, not terribly bad, but they described the water coming up to the sandbar creating an undercurrent, which carried them out into the water," Tim Walburg, Chief Deputy with the Lake County Sheriff's Office, said.
But Mark Smedsrud with Games Fish & Parks says because they were on a lake, that "undercurrent" that pulled on the boys was probably more like turbulence created when the wind kicks up waves.
"In our inland lakes here in eastern South Dakota, I don't think there'd be a true current in the lake, like you would see in a river," Smedsrud said.
In the hour leading up to the accident, there were 20 miles per hour wind gusts reported in Madison.
"Definitely the wind can cause water to move across a point, especially if you're coming from a deeper water area up onto a shallow point," Smedsrud said. That change in depth is exactly what you'll find at Walker's Point at Lake Madison. Going out on the sandbar is like walking along a wide ledge. There's a steep drop off on either side of that shelf, which you may not see. The water moves over and around the sandbar.
"As the wave action occurs, it somewhat pushes the water. It's like viewing it, pushing up a hill, because you're going from deeper water up to shallow," Smedsrud said.
That can create hazards even when you're staying close to the shore.
