Quite an Unusual one

[jimbo_cnc]

Another thought:

Are the bridge deeper than the side pools? If they are the same depth then the shrinkage should not cause flow, but if the bridge pools are deeper, then they will get inflow like you experienced because they will slump more.

Narrow deep bridges will suck less from the rails than wide deep bridges. So narrower would seem a good choice.

And all this assumes vertical walls.