From their website:
Geomorphic Felt River System: The river systems sets are geomorphic, which allows you to make rivers in endless lengths and gives you an endless array of layout possibilities.
Hotz Mats Felt River Systems sections come ready to use: no cutting or painting is required, they lay flat on any gaming surface (works best on gaming mats and/or flat carpets), are easy to transport and store, folds/creases that may develop during storing are easily and quickly ironed out, designed to last many years of normal gaming use and look great! This product is designed to be light weight, take normal gaming punishment and should last you a very long time.
Felt River System Sizes: The Felt River System come in 1.5, 2, 3 and 4 inch widths, which are the most common widths required for most games. Other sizes will be released at a later date.