DEV FROM CLRNC RTE - ACR DOES NOT FOLLOW THE NOISE ABATEMENT RTE UNTIL PROMPTED BY DEP CTLRS.

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GIVEN TKOF CLRNC RWY 36 DCA WITH INSTRUCTIONS TO MAINTAIN VISUAL SEPARATION FROM AN MD-80 THAT JUST DEPARTED. AFTER TKOF GIVEN A CLRNC TO TURN L AND JOIN NOISE ABATEMENT DEP (REDUCE PWR AND FOLLOW POTOMAC RIVER NW). SWITCHED OVER TO WASHINGTON DEP CTLR WHO ASKED IF WE HAD THE MD-80 TFC IN SIGHT AHEAD. REPLIED AFFIRMATIVE. CTLR QUESTIONED IF WE WERE OVER THE RIVER. WE REPLIED AFFIRMATIVE. A DIFFERENT CTLR IMMEDIATELY QUESTIONED IF WE WERE OVER RIVER. AGAIN REPLIED AFFIRMATIVE. THIS VISUAL DEP IS DIFFICULT TO DETERMINE EXACT COURSE (FOLLOW RIVER) DUE TO HIGH NOISE ATTITUDE AND POOR VISIBILITY THROUGH SIDE WINDOWS. TYPICALLY THE ONLY WAY TO EXECUTE IS TO MAKE THE RIVER DISAPPEAR UNDER THE ACFT. IF THE RIVER BECOMES VISIBLE YOU MUST BE OFF COURSE. THEREFORE IT IS IMPOSSIBLE TO POSITIVELY VERIFY IF OVER THE RIVER AND ON COURSE VISUALLY. THIS DEMANDING VISUAL DEP COUPLED WITH INSTRUCTIONS TO MAINTAIN VISUAL SEPARATION WITH TFC AHEAD AT NIGHT MADE IT POSSIBLE TO BECOME SLIGHTLY OFF COURSE. TO THE BEST OF OUR KNOWLEDGE WE COMPLIED WITH ALL PROCS AND CLRNCS. THE ENTIRE OP AT WASHINGTON NATIONAL ARPT IS MARGINAL. CTLRS MUST HAVE HAD DOUBT AS TO OUR EXACT POS. ONCE WE CONFIRMED WE WERE ON COURSE OVER THE RIVER THE SECOND TIME NO FURTHER COMMENT FROM THE CTLRS. CONTINUED FLT WITHOUT INCIDENT.