How to make water move in a square bed
This is an interesting question. Actually, you can have a squared waterfall with the top trim being all the same height, however, the bottom has to be an incline leading to a sump pit where you can install a small water feature submerged pump. I'm not much at drawing pics, maybe one of my fellow SE buddies can help me out here.  Let me try to explain the theory. Even though you want the shape to be square, think of it as a straight sluice. You will have a pump at the lowest point, and recycle the water to the highest, starting point. So if you build a square canal, there must be a start and an end. The barrier between the "start" and "end" could be very thin, maybe a piece of plexiglass. The difference in height between the start and end does not have to be much, but practically speaking would depend on the dimension of the overall square. So if the start of the flume was 6 inches deep(for example). then the end of the flume/ pump location, could be 12 inches. Since water is going to seek it's own level, the volume of water in the feature has to be a bit less than is required to fill the entire feature to the top.  So let's see how it works: Water from the lowest point/pit or reservoir is pumped up over a barrier to a shallower starting point, raising the surface level at the "starting point. Gravity and the flow created from the pump, sends the water downhill along the sluice. As the water seeks a level state, it flows back to the pit/reservoir where the pump lifts it to continue the cycle.