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.
All of the Cityscapes of The Flash, as far as I have seen, I've seen every one< are shots of Portland Oregon. I know the Skyline, because I've lived there for 50 years. Its funny tho', The round Starlabs building is projected into the shots, and it jumps from one side of the river to the other in other shows.