There was a time when felt was the most popular material for boot soles. They aren’t ideal for hiking long distances or walking in muddy streambeds, but they offer great traction on slippery river rocks. But it’s now known that the felt can harbor invasive species, such as algae or parasites, that can then be transferred to the next river you fish in, with potentially devastating effects on stream ecosystems. Some states ban felt outsoles, so check the regulations in the states you’ll be visiting. Rubber boots aren’t great on slippery rocks, but boots with deep treads are better for muddy conditions, and some are comfortable enough for hiking to more distant fishing spots. They’re very easy to keep clean and much less likely to spread invasive critters. Note that you can change the outsole on some boot brands, using rubber or felt depending on the conditions.