RedactedNarrative: Operator was traveling up stream leaving Winneshiek Slough and entering the main channel of the river.  At approximately river mile 667 operator attempted to drive over a 3-4 foot wave.  Boat was traveling over the wave, the water spilled over the gunwale on the stern, starboard side.  Operator attempted to keep driving but the water filled the boat too quickly and shortly after the boat completely sank.  Improper loading was a cause.  Operator stated that the boat was on unique design.  The boat was narrow and long and was a little unstable.  Operator stated that when there is only one person in the boat, the bow will rise and the stern will drop when underway.  Operator stated that he placed his gas tank in the bow of the boat as well as an anchor inside the bench seat in the boat to reduce the rise.  The anchor is attached to the boat with rope.