To answer your question, the core trench isn't necessarily set by the lowest point. The key (forgive the pun) to a keyway/core trench, is to create the best "waterproof" barrier of clay as you build the dam. That foundation can rise, undulate, or fall, and likely does, since nothing about soils sits on a flat plane. I've seen a few cases where the core trench was simple, until the earthmover figured out a creek bed was way deeper than it should be. Another was a field that had been manipulated decades ago by straightening a creek channel. Under that field sat the old creek channel, winding beneath that field. That core trench went up and down, all over.
