The self oxygenating Dart

One of the curious phenomena one encounters on the tidal Dart is the lines of bubbles, usually at the edge of the deep channel but often also in the shallows at the very edge of the river.

This is a line of bubblers near Sharpham quay, the foam being washed downstream by the falling tide.
Sharpham bubblers

My guess is that they are caused by air entering porous gravel at low tide. When the water rises again, the air in the gravel is trapped against an impermeable cap of mud, which is breached at intervals to allow the bubbles to rise. In some places the mud has a hard consolidated layer, seen at the bottom of the next picture, which is penetrated by the air channels.
Bubblers under peat

If any reader knows for certain how this phenomenon arises, please post a comment.