Newbie question. During terrain placement the defender rolls to place a river on the North East quadrant. Are these three placements allowed? "A River must run from 1 battlefield edge to a different battlefield edge"
And when the invader is choosing the base for deployment which sides are not opposite a waterway?
As long as "It cannot start or go within 4 BW of any battlefield edge except the 2 edges it flows from and towards", then yes, all three placements are valid. I'm especially fond of the "bending river" for "Tropical" armies, nifty.
Concerning your other query, as waterways are a totally different kettle of fish, and assuming the absence of a road, the invader could choose any side.
Arnopov is right that a River is not a Waterway...they are completely different. An invader cannot choose a base-edge that is opposite (i.e. facing) a Waterway. (See page 8, first paragraph)
And Haywire is also right about linear features can only extend into one adjacent quarter. (See page 6, fourth paragraph)
Also note that bendy rivers cannot be longer than 1½ times the distance between the ends. (I use a piece of string draped along the river, then stretch-out the string into a straight line to measure it)
Oh, and note also there is a bit of a contradiction when using Roads and Waterways together.
Roads cannot end on a Waterway, but they can end on a BUA that is touching a Waterway edge. Deployment on page 8 first paragraph says “The invader selects the base edge. If a Road crosses the battlefield, one of the intersected sides must be chosen, otherwise any edge that is not opposite a Waterway”.
So if a defender places a Waterway, then has a BUA touching it, they could place a Road to end at the BUA. That means the invader has no choice but to take the Road end that intersects the table edge as his base... ...which is opposite the Waterway...which he is prevented from selecting! 🤪
The easiest solution to this paradox is to take the rules literally, and follow them word-for-word. “If a Road crosses the battlefield, one of the intersected sides must be chosen...” A Road ending at a BUA does not fully cross the battlefield, as one end of it does not touch a table edge. Therefore the invader can ignore the Road limitation and choose any edge (except that opposite the Waterway).