a video i shot while up in sw alaska...
Mitchell Bay, Alaska 24' ebb tide running through the narrows

NOAA: Tides are very long-period waves that move through the oceans in response to the gravitational pull of the moon and sun. Tides originate in the oceans and progress towards the coastlines where they appear as the regular rise and fall of the sea surface. When the highest part, or crest, of the wave reaches a particular location, high tide occurs; low tide corresponds to the lowest part of the wave, or its trough. The difference in height between the high tide and low tide is called the tidal range.