The river changes direction and actually flows upstream for quite a few miles when the tide is coming in (rising). The sheer volume of water that turns around on a dime every tidal cycle in the Columbia is truly amazing. Currents in the main channel are about 7 knots off Astoria, which can overpower many small boat engines. Currents are quite a bit slower in the shallows near shore, and around the islands and sloughs off Brownsmead. Lots of great kayaking territory there.
Red and green buoys mark the shipping channel, which is nearest the Oregon side of the river off Astoria. Red buoys are on the south side of the channel, green on the north. The way to remember this is to think of coming toward Astoria from the ocean. There's a rule that goes: Red Right Returning. It means when you are coming into a harbor from out at sea, the red buoys will always be on the right. This is true for coastal waters and piloting. Inland navigation rules (on the Great Lakes for instance) are different and the same rules may not apply.
Have a great weekend. Happy Ship Watching!