B737-300 DRIFTED OFF COURSE DURING THE RIVER VISUAL APCH AT NIGHT INTO DCA. WINDS MAY HAVE DRIFTED THE ACFT TO THE L WHICH THE PIC; PF; FAILED TO NOTE. APCH CTLR CAUGHT THE SIT PRIOR TO ACFT FLYING OVER P56. CREW WAS FIXATED ON THE DARK AREAS THOUGHT TO BE THE RIVER.

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FLT JUN/XX/97; APPROX XY38 EDT. ACTIVE RWY IN DCA RWY 18. RIVER VISUAL APCHS IN USE. UPON ARR IN THE DCA AREA; WE WERE DSNDING DOWN TO 3000 FT WHEN THE CTLR ASKED IF WE HAD THE RIVER IN SIGHT. MY FO SAID HE HAD THE RIVER IN SIGHT AND I COULD SEE THE RIVER TO MY L; WHERE IT WAS DARK; AND TO MY R; WHERE IT WAS ILLUMINATED BY THE LIGHTS OF THE BRIDGES. NOSE ANGLE AND ALT OBSCURED MY VIEW OF THE DARK BEND IN THE RIVER AHEAD OF US. THE CTLR GAVE US A HDG OF 150 DEGS TO PICK UP THE RIVER. THE FMC HAD THE 6; 4; AND 3 MI WAYPOINTS SET. THE FIRST; THE 6 MI WAYPOINT DROPPED OUT OF THE COMPUTER; INDICATING WE WERE PAST THAT POINT. WITH MY EYES ON THE LIT PORTION OF THE RIVER AHEAD; I CONTINUED ON THE 105 DEG HDG; THINKING THE DARK AREA AHEAD WAS WATER; AND BEGAN A DSCNT TO REACH THE 1800 FT AGL (DCA RWY 6 RECOMMENDED; ALTHOUGH 1200 FT WAS THE RECOMMENDED FOR THE NEXT ALT; THE DCA RWY 4 -- I WANTED TO BE OVER THE WATER BEFORE DSNDING BELOW 1800 FT). THE CTLR THEN WARNED ME THAT I WAS PROCEEDING TOWARD THE PROHIBITED AREA. AT THE SAME TIME; THE FO SAID THAT THE RIVER WAS TO THE R. I INITIATED A SHARP TURN TO THE R AND REGAINED THE RIVER JUST INSIDE THE DCA RWY 4. THIS; OF COURSE; NECESSITATED A SECOND SHARP TURN TO THE L TO REINTERCEPT THE RIVER AT THE KEY BRIDGE. THE LNDG WAS UNEVENTFUL. I BELIEVE THAT NEITHER OF US HAD THE DARK BEND IN THE RIVER IN SIGHT. I DON'T KNOW WHAT THE FO WAS LOOKING AT; BUT I THOUGHT THAT THE DARK SHORE NEAR THE PROHIBITED AREA WAS RIVER. WE THEREFORE BOTH MISSED THE R TURN THAT WOULD HAVE KEPT US OVER THE RIVER. DUE TO MY FIXATION ON THE LIGHTS OF THE BRIDGES; I THOUGHT I WAS ON THE CORRECT FLT PATH; ALTHOUGH I HAD PASSED THE DCA 148 DEG RADIAL WITHOUT NOTICING. (RAW DATA SET IN VORS.) I HAVE BEEN INTO DCA MANY TIMES. I WAS TOO COMPLACENT AND FAILED TO PAY SUFFICIENT ATTN TO MY INSTS ON THIS VISUAL APCH. THIS IS ESPECIALLY IMPORTANT AFTER SUNDOWN WHEN IT IS TOO BLACK TO SEE CLRLY. THE CTLR HELPED THIS SIT CONSIDERABLY.