Zones 5 - 6
The nitrogen flow appears to be running aground as its moving west and north, The red tholin land ice is thick with high walls on its eastern edge, The icebergs seem to be moving in from the south,
The largest icebergs are pressing against the east wall of land and being funneled northward. As the icebergs travel north, they encounter shallower land this in turn grinds the icebergs into smaller pieces from below.
The ocean of nitrogen is deep near the south east walls of the land but migrating north the land ice becomes shallow like a beach. As the icebergs are compressed onto the shallower land, they tumble and grind into smaller and smaller chunks.
This is an elevation map of the same area.
The most important information gleaned from this observation is that larger icebergs are in deeper nitrogen fluid, smaller icebergs are in shallower nitrogen flow.
This means we can guestimate the depth of the land below the liquid by the size of the icebergs above (important point).
Remember this point as it will shape important future interpretations of SP.
The height of the icebergs on the surface give an indication of the depth of the fluid within which they are floating.
The East Wall
The image to the left is showing the water density in different regions of Pluto. Dark blue is where the western edge of the nitrogen flow meets the land's eastern ice wall at its strongest and likely tallest point.
The eastern side of Sputnik Planum (right side of heart) is a shallow terrain and is covered in nitrogen flow and clustered icebergs. The cleavage of the heart is also dark blue and it is resisting the onslaught of the nitrogen flow.
Here is a closer view of the eastern land wall.
The flow is emanating out from a central area, it has collided with the land ice wall and is spreading out north and south to find lower land to crawl onto. There is also evidence the fluid is broiling up along the edge of the land ice and traveling into the deeper area of SP. In other words things are moving in multiple directions.
The two most significant indicators of a wall being present are the dense water accumulation (above image) and the large icebergs which infer deeper fluid.