An interesting issue I have come across here is that the use of felt soled boots is not considered very eco-friendly. The argument being that they absorb and hold moist the various organisms from the river and transfer these to other rivers. This is a particular problem here with an algae called Didymo or river snot. The fly fishermen allegedly fish the rivers on the South Island where this is prevalent and transfer it over to the North Island in the felt on their boots. The only way to prevent this is to boil the felt before you go into any new rivers.

Something of a controversial theory! I am throwing it out to see if we can get some other opinions.