Here is one of the answers:
New Product Review: Svelte - An Alternative to Felt on Wading Boots ...
https://www.fieldandstream.com/.../n...ve-felt-wading...
Mar 3, 2011 - We all know that the days of felt soles on wading boots are numbered... at least in some places. Citing felt's role in the transport of invasive species that can harm fisheries, wildlife agencies are encouraging (and in some cases mandating) the use of non-felt boots. Thing is, the jury is still out on the grip and ...