The evening was still and had good clouds. It made the photo because without the clouds, there is nothing interesting here. The sun would be overpowering and glaring. The water would be flat and boring, and the sky would be a complete white-yellow eggy mess.
The aqueduct is the largest and best looking one on the C&O Canal Historic Park trail. BUT... photographically speaking, without natural conditions, it's pretty bad to shoot, from any angle.