The infamous Swellies is well known on the Menai Strait. The precise point where the flows meet (Swellies) changes depending on the size of the tide but is always found between the two historic bridges at the narrowest point of the Strait.

As the two flows meet the southerly flow starts to loose its strength and for a short moment the opposing flows almost equal each other out.

At this point the current appears to stop for a few moments and becomes slack, before the direction of flow quickly changes and starts to build in a southerly direction. Once they meet the tide continues to build for another two hours. The southerly current then picks up great speed and heads south to the Irish sea. 

This unique tide appears to wash out the majority of incoming pollutants to the open ocean. However there are key land hooks in the south which mean pollution can be potentially pushed to the land edges and remain in the Strait for some time. The moment in the middle where the tide becomes slack can also leave sediment, nutrients and pollution to drop to the floor between the bridges.