The Dora Baltea gathers itself from glaciers under Mont Blanc and Monte Rosa, and by the time it reaches the middle valley below Aosta it has become a proper river — wide, deceptively even-paced, the kind of water that looks straightforward until you've waded a hundred yards of it. The no-kill reach through Chambave, Fenis, Nus, Saint Marcel and Verrayes runs for about nine and a half kilometres over a bed of sand and bedrock sills, and it holds the valley's best case for wild trout: brown trout and native marble trout share the deeper glides here, with grayling present on the wider river system. A long-running tagging study follows both trout species through this water, which tells you something about how seriously it's taken. Everything goes back — barbless hooks, three flies maximum, catch and release non-negotiable. The river runs milky with glacial melt through high-summer afternoons; fish it early, or wait for the settled water of autumn.

• Granite schist