Earlier I asked a few questions about one particular part of my setting, which is an Earth in the immediate aftermath of a supernatural apocalypse that made the world a gridded globe of 2x2 mile "squares" (pillars getting larger going up and smaller going down, along latitudinal and longitudinal lines) that are each subject to a different supernatural gimmick. The first of these "squares" that the heroes have to deal with has invisible walls stretching 100 feet (30 m) off the ground (it's in a flat stretch of a suburban town), and rain is constantly falling at the incredible rate of an inch per hour. Upon closer inspection it's later revealed that the rain isn't actually falling from clouds, but rather appearing in midair slightly above the height of the invisible walls.
This isn't infinitely generating water that will eventually cause the world to be completely flooded millennia down the road or anything; another square somewhere in the ocean is providing the water, though just the pure H2O and not the salt or other stuff mixed with it. But it will keep filling with water forever, and overflowing off the sides when it finally gets filled to the 100 foot (30 m) line roughly 50 days later.
My previous question dealt with how long it would take before that water spilling off the side would become drinkable. My new question has to do with what exactly that eventual four-directional waterfall would do to the surrounding terrain. My primary guesses are that it'll either forcibly carve out one or more new rivers, or turn everything south of the thing into mucky swampland, but I really don't have clue one, so I thought I'd ask here.
Some extra info: the setting is a fictional New Jersey suburban town called Silverwood. The 2x2 mile area of town "The Fishtank" is in is almost entirely flat, but to the south the land slopes somewhat downward, and to the north the land slopes upward. The area is largely developed, with plenty of trees but nothing resembling "wilderness" for miles around.
Once this flooded barrier starts spilling water over its invisible walls, what will that water do to the surrounding area?