Here is an easy reference for determining when the approximate time is for slack tide at Reversing Falls on the Saint John River. Slack time provides an approximately 20 minute window for traversing the passage.
These times are based on:
- Low slack, add 3 hours and 50 minutes to low tide
- High slack, add 2 hours and 25 minutes to high tide
The source tide data is from Fisheries and Oceans: http://tides.gc.ca/eng/station?sid=65
Note: Fisheries and Oceans now has a 7 day Slack Time report on their tides page. It now has a feature for looking forward for the prediction. That should be your definitive source.
Also Note: Actual transit times will vary depending on the water level of the river. If the river is exceptionally high or exceptionally low, you will need to adjust your time. It is usually better to arrive a bit early and wait for it to flatten out. The Fundy Traffic VHF channel can also be a good source of information on transit times.
Transit times are current local time with DST applied.