The pilot was on a local, personal flight in an experimental airplane that had accumulated 32 hours total time. It was powered by an engine that produced 85 horsepower. While in cruise flight approximately 600 feet above ground level he heard a loud bang and the airplane entered a right turn. The pilot used aileron to counteract the turn; however, the airplane descended into the reservoir below. Post accident inspection showed that the rivets securing the rudder pedals had sheared. During the impact sequence, the fuselage sustained structural damage. According to the kit supplier, the rudder pedal horns are secured using three rivets per pedal. The kit supplier indicated that the rivets could be put under excessive stress while operating the airplane over gross weight and/or powering the airplane with an engine that produces over 65 horsepower. Although there had been no other reports of this kind to the kit supplier, following the accident he suggested to his customers that they check the security of the rivets prior to each flight and if they are loose, replace two of the three rivets with two bolts provided by the kit supplier. All future rudder pedals provided by the kit supplier will be equipped with the bolts.