The upper Doubs is one of the most argued-about rivers in Europe, and deservedly so. It rises in the French Jura, runs north to Switzerland, forms the border for thirty kilometres, and then re-enters France to begin its long loop back south. On paper it should be a world-beater — limestone bedrock, cold plateau springs, prolific hatches, legendary trout and grayling stocks, and a century of serious fly-fishing heritage. In practice the last twenty years have been hard. Low summer discharge, thermal stress, agricultural and urban pollution, cormorant pressure, and a mysterious recurrent saprolegniosis outbreak have all taken their toll. The fish are still there and the river is still beautiful, but the numbers are a fraction of what they were and the management is a cross-border argument between two countries and several interest groups. If you go, go with realistic expectations. Spring is the honest season. Bring your respect for a recovering river and don't kill anything. When the upper Doubs is on — a cool morning in May with olive duns drifting through a glide — it still reminds you why people cared about it in the first place, and why they're fighting to get it back.

• Limestone