This is an image best viewed large since the article is all about the details here.
Below is a ‘trunk’ I have tried to describe as part of multiple other articles. Here a Black-crowned Night Heron sits on a supporting pilon. A favorite fishing spot.
Water flows through the wood gate on the far right side. Alligator warnings and other rules are posted on it.
The main part of the structure, a huge wooden rectangular box runs underground, right through the middle of this dike.
Another wooden gate is on the other side. Opening and closing each side regulates the water flow back and forth. The marshes all have trunks. You could open them in a row of connected dikes. That means water flows back and forth, through all the trunks, to regulate hundreds of marshes. A clever system that was first in use along the west African coast, hundreds of years ago.
A small Heron like below, or Egrets, can be found standing right in that same spot. A few times I was sure it was the same Night Heron until I noticed a second one watching from nearby.
It’s very simple to look below, spot a small fish, then drop down and grab him. The water is deep, may have a current, or more likely…an Alligator. They like fish too.