The tide is the daily rise and fall of sea level. Like all rivers,
the Thames flows down to the sea. But when the tide rises in the sea,
the sea water floods up the river Thames. On the Thames in London,
the tide rises and falls roughly every 12 hours 25 minutes. So every
day, the two high tides are about half-an-hour later than they were
the day before. At Greenwich, the Thames can rise or fall by 16 feet
with the tide.