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I live in Totnes, a UK town on the tidal river Dart estuary.

There is a perfect height of water for putting my canoe into the water, at a set of steps near a local pub. Being tidal, this perfect water height occurs several times per day.There are no measurements on the steps and the river bed is mostly very silty and soft. But how might I work out when that time of water height for canoe launch will be, say, a week in advance?

I have tide prediction tables and they are available online, and they show tide height above ‘ chart datum’/ ‘Lowest Astronomical Tide’. If I go down to the steps, is there an way for me to work out at what exact water height above chart datum is ideal for my canoe launching? Other than going down there at the perfect time and looking online what the current water height above chart datum is?

That is, is there any sure fire way to look at a body of water /river bed/ harbour/waterfront/ these steps, and know where chart datum/LAT is?

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You've just missed your chance for one nice simple check, because we've had really big spring tides recently and you would have been able to see the LAT (or near enough for paddling), and from the right vantage point you may have been able to compare it to your launch point.

But you can find past tide times (e.g. on tidetimes.org.uk), and check the tide height and state for a time and date that was good. So if you know fairly precisely when you've been before (do you log your paddling on Strava?) and whether it was good, you can get a pretty decent idea of the depth. Totnes has fairly simple tides, so half way between high and low in terms of time is also half way in terms of depth. While that doesn't hold for other times in the tidal cycle, it may be near enough for your purposes.

Going a step further, you don't just need sufficient depth*, but whether it's ebbing or flowing, depending on where you're going and how long you'll be. In particular coming up the estuary on a rising tide is good - I've paddled up from Kingswear to Totnes in a sea kayak, for example (over 10 years ago, but I think we got out at Steamer Quay).

You can work out a practical datum for your own purposes. With steps, just count them from the top or some clear feature, and reference those to tide times. You could measure a step, then you'd only have to visit once to convert to metres above datum.

So if you know (for example) that it was 4 steps down at 6pm today, roughly half way up a rising tide, you know you need to plan for something like 1.5m (or half way up the tide, again). But that's when the flow is strongest, i.e. when it matters the most which way it's going.

* Too deep is also possible, but mainly at spring high tides; there's often an alternative landing, but I can't quite picture it in Totnes

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  • Adding to this, if you can work out the best time to launch on a rising tide, so that when you finish the tide hasn't turned, that will be safer on an estuary. I've been caught out by trying to paddle back to my mooring point on an ebbing tide. It was stupendously difficult.
    – Weather Vane
    Commented Mar 15 at 23:28
  • @WeatherVane that's true assuming you paddle downstream first (as you would from Totnes in the estuary). But if you wanted a long paddle, like Kingswear and back (or more than about halfway if you were taking it very easy), you might want to set out as the tide was falling, have lunch at slack water at the far end, and return on a rising tide. That's what I was getting at in the third paragraph
    – Chris H
    Commented Mar 16 at 10:36
  • Thanks for comments! Yes these are all good ideas, I will look up Strava. Yes, on the super low spring tide recently we actually had a low tide of -0.14, so 14cm below chart datum, which I don't quite understand, but hey ho. Like you say, sounds like i will just have to work out some known identifiers from checking tide heights digitally and looking where the water is at that moment around known features down at the water. Thanks for eveyones input!
    – Captainpadlock
    Commented Mar 18 at 15:26

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