Bore tide, which basically is what you get when the incoming tide is funneled into a narrow area, producing a distinct difference between high and low tide, which is a wall of water, with tremendous force.
Turnagain Arm is approximately 45 miles long.
The shot above is the Turnagain Arm of Cook Inlet, Alaska which looks like a harmless shallow body of water, and its mud flats may look good for walking, but they’re not. Just like quicksand, the flats can grab and hold the unwary .
With some of the fastest tidal changes in the world, Turnagain Arm’s famous “bore tide” comes in - with a unique and deadly 6-foot wall of water, traveling at 15 mph,there’s no escape. Bore tides come in along Cook Inlet after low tide in a rolling wave. You can often hear a bore tide before you see it. It has a low, thundering roar, and is caused by narrow, shallow channels. The tide takes around 5.5 hours to get to the end of the inlet.