1966 El Camino Floorboard

I'm putting in a new carpet on my '66 Camino. The passenger floor is the strangest configuration...almost like it's been hit. It's got a funny trough at the front where the floorboard starts moving up towards the firewall. This trough is a big water catch. It really seems stupid for GM to have built the floorboard this way as there is no place for water to go but sit there and rust. There is no other signs of my truck being hit so I'm confused about the configuration. Can anyone comment?