It’s a tidal bore called “Mascaret” which can be 2 km long !!! try it! It works at each big tide (coefficient more than 95). It’s better if it hasn’t rained for a long time because the river is lower. So the best season is during September. You must know precisely the time of the change of the tide. Take your biggest board: you’ll have no chance with a shortboard. Specialists have built very long boards (3,5 to 5,5 m!). Go in the water around 30mn before this time. You have to paddle towards a big red buoy on your left (distance 1,2 km). Sometimes the Mascaret comes 10mn before or later. You will see the Mascaret coming in: it’s very impressive. Be careful: you’ll have only one wave. Don’t miss it. If you miss it, don’t worry, the current will get you back to the harbour. Good luck and have fun.
It’s crowded: kayack, longboard. But the atmosphere is very relax: it’s like a big party. Comes with friends.