Depths

Gauges – how deep is the Tidal Trent?

In coastal waters, a navigator would consult his chart to find a depth on his course line, then consult “Tide Tables” to see what additional water is predicted to be brought in by the tide at particular location at a particular time. Add the charted depth to the predicted tide and the navigator will know how much water they will have to “float his boat”, at that location, at that time!

Unfortunately we don’t have proper charts above Gainsborough, nor do we have “Tide Tables” with predicted heights of water. (The Admiralty give up trying to predict the low tide heights above Keadby).

The following alternative is provided with our best intent, but MUST not be relied on. For more accurate information, contact the relevant navigation authority (CRT above Gainsborough, ABP below).

The approach consists of two parts:

• An estimation of the minimum depths, at a normal low tide, along the “Red Line” on the Boating Assocation using information from CRT’s “non navigation” survey of March 2022, and from a limited number of corrected “sonar runs” by amateur navigators. In theory we believe this is 1.6m between Cromwell and Gainsborough, but our estimate is that the minimum depth in some places is 1.2 metres.
• An estimate of the water level relative to the normal low tide level. We can estimate when each high and low water will occur, but we cannot predict the actual height of that tide in advance as it can be hugely, and almost randomly, affected by the amount of fresh water flowing down the river from inland. The website below shows this height in the final (right hand) column.

So, to give a couple of examples, for Cromwell to Gainsborough:

• If the water level a given location is 2.0m above normal low tide, then there should be at least 3.2m of water there (2.0 + 1.2 = 3.2)
• In a time of drought, if the water level is -0.3m, then there could be as little as 0.9m of water in which to float your boat. (-0.3 + 1.2 = 0.9)

This link takes you to our page that uses the four “Flood Gauges” provided by the Environment Agency on the tidal stretch. The final column gives an indication (with a one hour lag) of how much “extra” water there is, above the normal low tide.

A brief explanation of each column in the table follows …

STATION		Self explanatory – but each name can be “tapped” to take you direct to the actual EA Gauge. The “North Muskham” Gauge is different in that it is “non tidal” but is the gauge that gives us the advisory “Flood/ Fast Flow Warnings” for the whole tidal section. The colour here changes automatically
GMT/BST		The time of the last reading this software is considering. Unfortunately, while the Gauges usually update every 15mins on the tidal section, it takes time for that information to become available, so this information can be up to an hour late – Keadby in particular has potentially an even bigger delay. If you want the most up to date number, go to the EA Gauge…
GAUGE		This is the actual reading on the EA Gauge. It is linked to a local “datum” that is NOT the depth of water at that location.. “” +/-” – is the indicator of a rising or falling water level at this location (You can follow the progress of a tide as it moves upstream!)
CHART DATUM (CD)		This figure doesn’t change, and is the correction that has to be applied to each location to convert the Gauge Readings to “Height above Chart Datum” – the tidal readings that navigators require.
ABOVE CHART DATUM		Is the height of tide and fresh at this Gauge at the time shown. Note that Keadby frequently goes “negative” which suggests the “Chart Datum” here is incorrect, in isolation this is of limited concern in these lower reaches. Other locations will go negative only when river levels are low, and may be cause for concern if your boat draft is 1.0 metre or more, or if this number becomes significant.
PREVIOUS LOW +/- CD		The most useful colum for day to day river use! This is the time (note it’s in GMT, so add an hour in summer!) of the last “low water”. You will see the “low water” moves up the river from Keadby to Torksey and on to Carlton – but you may find when tides are weak, or “fresh” is high, Carlton gets seriously out of sequence! The last “Low water” here can generally taken as the “amount of Fresh at Carlton” which reflects, but may not be the same as the “Fresh at Cromwell ” supplied by the lock keepers. Finally…. No… They don’t all have the same amount of water “above datum” on each tide! AND…. this is the time of the lowest level on that tide….. It is NOT necessarily when the tide turned. The “First Flood” when the lock keeper will let you out to go upstream can be significantly later than the “low tide” at all these locations – but the further up the river you go… The bigger the gap MAY be

Complicated? Maybe. But

• if your boat draws 1.0m or less, you will only need to even think about this if we have summer droughts again – and it is to be hoped that the lock keepers will be able to give warnings of such occurrences just as they do “flood” conditions.
• If your boat draws above 1m – you might like to read this again, especially for trips above Gainsborough, and bookmark this page.

CAUTION – If you use the information provided by the EA gauges via any other link, the “heights” given DO NOT tell you the depth of water above chart datum or even above the river bed – the number is much too large! They measure water levels relative to a different datum level (eg Ordnance Datum Newlyn used by the Ordnance Survey). These gauges are not there for navigation, and may go into error with no warning.

NOTE: All information given here is given with best intent, but MUST not be relied on. For more accurate information, contact the Navigation Authority.