I agree it was "just for effect". If the pipe had had a DIP in it (and Andy had been operating at the "dip"), then pressure could have built up to create the fountain. The dip would have to have been at least as deep as the fountain was high. Problem with this is that the pipe would have remained filled and been unnavigable. Yes a pump in operation at the exact moment of opening could have produced the fountain (and then astonishingly conveniently ceased five seconds later) -- but it seems clear the system was "gravity fed" and not pumped (therefore the pipe carefully laid without dips). There is no conceivable way for there to have been fluid pressure sufficient for the fountain, AND then for the pipe to be mostly empty. The "shock value" was for audience sympathy ("EEEWWWWWWW!"), most not realizing it was after all just water, sawdust, and chocolate syrup...
I want to know how he knew there was going to be a STORM that night -- escape would have been so much harder without the covering thunder (and natural bathing). And how did he get to town, where did he change, when did he buy the car, why there weren't roadblocks, etcetera...
Let's pretend the town was near enough for walking (he had nearly all night), the rain ceased, and midway he found a closed gas station with an unlocked bathroom for changing & grooming. He visited one bank in town under the first alias ("Stevens") receiving enough cash to buy a car; which he did, under his second alias. He then drove to other TOWNS (the movie states "nearly a dozen banks in the Portland AREA"), therefore eluding road blocks. As he drove away from the first bank, he stopped long enough to buy the volcanic-glass-rock and the tin, and plant the already-written letter at the Buxton field (he had a lot of time to prepare as he worked in Norton's office). The glass rock could have been left 20 years ago as a memento, but he couldn't have been sure it would still be there; he likely knew where he could buy one to mark the tin. THEN he drove to other towns and emptied the other accounts as fast as he nonchalantly could, then he headed South to Fort Hancock in his new convertible with with the sun sparkling off the ocean in his eyes, cool breeze stroking his hair, and an incredible song of freedom singing in his heart. ;-)