During an aerial application flight over a river, the pilot was spreading seed when the airplane struck and severed all four wires of an electrical transmission power line. During the forced landing, the airplane impacted tall vegetation. The fuselage, empennage, and both wings were substantially damaged. According to the pilot and photographs of the area, neither the transmission wires nor the poles were visible above the tree line, adjacent to the river banks. The pilot stated that he had been warned about the transmission lines that extended over the river and should have located them before the application portion of the flight began.