The highest tide here in May is a 20.0 – which would put the marker at the very top of the picture. The lowest tide this month is a minus 4.7 – which would put the marker another 7 feet lower than it is in the picture.
It takes about six hours for that change of almost 25 feet.
Not all tides here are that extreme – a small high tide would be like a 12 or a 13 and a small low tide would be a 4 or a 5. A much smaller difference between high and low.
These two pictures were taken six hours apart. The high that day was around a 16 and the low was around a 2 – pretty average.
I’m sure you can see how this can cause transportation issues. A channel that is navigable at high tide might be impassable (or even walkable!) at low tide.
The tides also affect the flow of the water – the narrower or shallower the channel the faster the water flows.
In very narrow spots this ebb and flow can temporarily turn the ocean into a river and the ferry schedules have to account for his. There are some areas where the water can sometimes move too quickly for large boats to pass safely. In those cases the larger ferries have to time their passage for peak high tide or peak low tide (aka “slack tide”) when the water is still.
The newer “fast” ferries were built with strong enough engines to muscle their way through the narrows at any stage of the tide. The larger ferries can’t do this so their routes have to be meticulously planned – which is just one of the reasons that ferry schedules are not available a year in advance…
So if you are taking the ferries through Southeast Alaska and you have an extremely inconvenient arrival or departure time (like 2:45 a.m.) I hope you can understand that we’re working around Mother Nature.
Like I tell everyone that gets frustrated by the complexities of the ferry schedule, “it’s part of the charm…”