A BELL 47G3B1 CRASHED INTO THE SOUTH SANTIAM RIVER AFTER CONTACTING POWER LINES WHILE ON AN INSTRUCTIONAL FLIGHT. THE CFI (HELICOPTER) PLT RECEIVED SERIOUS INJURIES AND THE PRIVATE PLT (AIRPLANE) HELICOPTER STUDENT RECEIVED MINOR INJURIES. THE HELICOPTER SUSTAINED SUBSTANTIAL DAMAGE. THE INSTRUCTOR PLT REPORTED HE AND THE STUDENT HAD BEEN CONDUCTING 'CONFINED AREA' LANDINGS AND HAD DEPARTED ONE AREA FOR ANOTHER. DURING THE FLT HELICOPTER WAS FLYING DOWN RIVER INTO THE SETTING SUN. THE CFI BECAME AWARE OF THE PWR LINES ABOUT 5 FEET AHEAD AND WAS UNABLE TO TAKE CORRECTIVE ACTION. THE CFI REPORTED THE POWER LINES WERE UNMARKED AND HIDDEN BY SUNGLARE OFF THE COCKPIT CANOPY AND SHADE FROM TREES ALONG THE RIVER BANK. THE HELICOPTER FLIPPED OVER AFTER CONTACTING THE WIRES AND FELL INTO THE RIVER. THE FLIGHT WAS CONDUCTED ABOUT 50 FEET ABOVE THE RIVER.