River tide: how far up a tidal river the tide reaches

How to read river tides properly

1. Find the tidal limit. Every tidal river has a point above which the tide no longer reaches, usually a weir, a lock or a natural fall. Below it the water rises and falls; above it the river only flows one way.
2. Expect the turn to lag. The tide takes time to travel up a river, so high water at a town twenty miles inland can be an hour or two after high water at the mouth. The lag grows with distance and is roughly consistent for a given spot.
3. Expect the range to shrink, then sometimes grow. Friction usually damps the range as the tide runs upstream. In a funnelling estuary the opposite can happen first: the channel narrows faster than friction damps it and the range increases before it finally dies away.
4. Add the river's own water. After heavy rain the freshwater coming down holds the tide back and lifts the low waters, so the observed levels sit above the prediction and the ebb runs harder than the flood. This is the single biggest reason a river tide misses its table.
5. Use the coastal station as the anchor. The published stations here are coastal and harbour stations. For a spot upstream, read the nearest published station and apply the local lag you have learned rather than assuming the coastal time.

Safety and the limits of a prediction

Common questions

How far up a river does the tide go?

It varies enormously with the river's gradient and depth. A steep coastal creek may be tidal for a mile; a large low-gradient river can be tidal for a hundred or more, all the way to the first weir or fall. The tidal limit is a fixed feature of the river, not something that changes with the day's tide.

Can a river flow backwards on the tide?

Yes, below the tidal limit and on a strong flood, particularly when the river's own flow is low. The incoming water can overcome the downstream flow and the surface genuinely runs upstream for part of the cycle. In flood conditions the river usually wins and the flow stays seaward while the level still rises.

Why is high tide later up the river than at the coast?

The tide travels up the channel as a wave and that takes time. The further from the mouth, the later high water arrives, commonly by an hour or two on a large river, and the lag is fairly consistent from day to day for a given place.

Is the tide table accurate on a tidal river?

Less so than on the open coast. The astronomical prediction is the same calculation, but river flow, rainfall and wind have a much larger effect in a confined channel. After heavy rain expect the low waters in particular to sit well above the published height.

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