The Cusancin is a short, secretive limestone tributary of the Doubs that rises at two resurgence springs at Cusance and runs for maybe fifteen kilometres before joining the Doubs at Baume-les-Dames. It has no business holding the quality of fish it holds. The water emerges from the Jura plateau cold and clear and stays that way through the worst of August, which is exactly when other Franche-Comté rivers are gasping. Wild brown trout and grayling are both present and both take fly life seriously. This is a proper spring creek — the kind of water where you can stand above a glide, watch three fish working different lanes, and take twenty minutes to pick the one you want to try first. The river is narrow enough that a wrong cast puts everything down, and the fish are educated enough that you won't get many second chances. Fish it when you're feeling patient.

• Limestone