Where does this reading come from?

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This reading comes from the OPW (Office of Public Works) gauge on the quay wall beside Athlone Castle, just upstream of the lock. It updates every fifteen minutes. Live readings for water levels along the entire Shannon can be found on the OPW Hydrodata website.
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The river is approximately the same level from Athlone lock all the way to the downstream side of Tarmonbarry lock. However, Athlone station generally reads a little lower than the other gauges on the lake, because the channel narrows where the lake meets the river above the town, causing the lake to be slightly higher.

 

How does this relate to the CSIG charts?
 
The reading on this page shows how today's level compares with the datum the CSIG charts are based on.
 
For Lough Ree, that datum is 5.5 feet over the cill of Athlone lock. It is close to the minimum historical level for this stretch. The level has dropped below it in only four years since 2003, and the lowest reading in the whole record, on the night of November 2nd, 2018, was 4 ft 11 in over the cill. The datum was chosen so that charted depths are almost always there, and usually a little conservative.
 
So when it says the level is nine inches deeper than the charts, there is about nine inches more water than the chart soundings show. When it reads the other way, there is less, and the shallow areas deserve more respect than usual.

 

How do I get the charts?
 
The CSIG charts are free to IWAI members. If you are already a member, register at chartssig.iwai.ie for access to the current charts for the Inland Waterways, including Lough Ree.
 
If you are not a member, join the IWAI here.

 

What do the classifications mean?
 
The coloured label is the same classification of the current level used on the OPW water level site. It compares the current level against all measurements this gauge has recorded since 2003.