It’s a real chalk-stream, originating from a very small lake. It contains a so-called “green trout”(Trutta macrostigma – a species living only here and other rivers in Central Italy).
Waders are useless, given its depth which obliges fishing from the bank a difficult task due to the presence of canes and weeds for the most part of the year. River Fibreno cannot be entered everywhere. You should know the exact spot and time to approach far from danger. Chalk-streams have specific features and river Fibreno has all. Including reactive fish.
Fish eagerly accept tiny spiders and bulky offers. Of course, not always. The point is to know when.