Question: The shows Arrow and The Flash take place in Starling City and nearby(?) Central City. My understanding (assumption) is that both shows are filmed in Vancouver, although each show has city shots depicting the (fictional) city where the show takes place. Arrow uses Boston for cityscape shots (no official citation, but I live in Boston and recognize the shots). What city is shown in the Flash's cityscape shots?

Answer: 
The opening city scene (river running thru the city with several bridges over it) of The Flash series is DEFINITELY Portland, Oregon. That's the Willamette River, Downtown Portland on the left, SE/NE Portland on the right. Scenes on the computer screens and city maps in the backgrounds are also of Downtown Portland. I live and have for over 40 yrs. I always recognize the steel and glass awnings (downtown Portland, Pioneer Square, Pioneer Place, etc). It is a beautiful city with lots of very distinct landscapes, streets, buildings, street signs, back alleys, etc.
The flash and arrow are both filmed in Vancouver BC. I've seen only Vancouver cityscapes on the flash that I remember. I live in Vancouver and the cityscape here is distinct and I can always recognize it. Star Labs is BC Place in Vancouver.