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 cityscape at around 29 to 30 minutes into episode 5 is English Creek in Vancouver. You can see Burnaby Mountain in the background. The bridge in the foreground is also unmistakable.

Later, when he carries Beth out of the city by running on water, you see him run under the Lion's Gate Bridge in Vancouver. Stanley Park is at the left end of the bridge, North Vancouver at the right end.

I haven't lived there for 35 years, but those two scenes are obvious, even at this remove in time.

Google English Creek and Lion's Gate Bridge for images to confirm.