Hi Mark.
Essex plays over a flat property. There's probably little more than 3-5 feet of elevation change across the entire 130 acres or so, naturally.
Donald Ross designed a series of "drainge swales" throughout the property, to channel water off the course, generate fill material, and create the illusion of some topographic relief. But the swales aren't really ditches, like Oakmont's ditches. At least not anymore. I think at one time they might have been though - ie. filled with a bit of water, and heavy grass.
What's most interesting about Essex' swales is the topographic relief they provide. You walk down and then up through a swale at 2, 7, 8, 13, 14, 15, and 18. These "dips" do create the illusion that the property isn't as dead flat as it actually is.