The lower Lys is canalised, navigable, and full of things that aren't trout. The headwaters are another river entirely. Above Aire-sur-la-Lys, up through Fruges and into the Ternois hills, the Lys is a proper chalk stream — small, cold, stable, hedged by willows — and it holds wild brown trout in numbers that surprise people who only know the canal downstream. Limestone geology keeps the water cool in summer and clear after rain; the hatches are what you'd expect from chalk stream water in this belt, mayfly and olives and summer sedges and the odd blue-winged in September. The fishable water is short and the AAPPMA beats are modest, but it's a reminder that the canalised rivers of the north once all looked like this. Some of them still do, if you walk far enough upstream.

• Chalk