One waterway that is definately unnavigable is the river Blackwater, which is a tributary of a tributary of the Thames. It rises somewhere near Farnborough Hants and rarely achieves a depth of more than two and a half feet. Last Friday I took a stroll along the upper reaches and was startled to see a little egret. These mediterranean birds looking like a small white heron have been seen along the south coast in recent years, but this was near Camberley! How soon before we see them on the canals?
The walk took an even more eventful turn just a bit later when I struggled past a bankside barbed wire fence and slid unceremoniously into the river. But like I said, the Blackwater has no depth and although a I got a wet foot, my knees never got wet :-)