I took this shot a couple if years ago, I was going through some vacation shots for my family when I came across that one. The name Douro was given to the river, which means golden, because of the city lights reflected on it and making it look like liquid gold.
“The Dom Luís Bridge (Portuguese: Ponte Luís I or Luiz I) is a metal arch bridge that spans the Douro River between the cities of Porto and Vila Nova de Gaia in Portugal. At the time of construction its span of 172 m was the longest of its type in the world.
The government held a competition for the construction of a metallic bridge over the Douro River on a site that was adjacent to an existing bridge that it would replace. Téophile Seyrig had engineered the D. Maria Pia Bridge project nearby, whilst working as a partner of Gustave Eiffel. He now took sole responsibility for the new, major Luís I Bridge. The construction was begun in 1881 and the bridge opened on 31 October 1886 (the upper deck; the lower deck opened in 1887).”