The Ain rises at about 700 m from a karst spring near La Favière on the Jura plateau and runs 190 km to the Rhône near Saint-Maurice-de-Gourdans, draining 3,630 km² of Jurassic limestone. The upper and middle reaches are partly-confined pool-riffle and bedrock runs on limestone cobble, punctuated by the Gorges de l'Ain near Corveissiat where the valley tightens to a confined slot. Below the Vouglans dam the river is profoundly regulated: the reservoir traps bedload, so the lower 40 km — a sinuous gravel-bed reach through Pleistocene glacial till — is sediment-starved, progressively incising and armouring since the 1970s as hydropeaking releases winnow sand and fine gravel further downstream. Mean discharge is roughly 130 m³/s. Slippery limestone slabs in the gorges and scoured troughs below dam outlets are the main wading hazards.

Wading: Wet limestone slabs in the gorges and scour troughs below dam outlets

On Jura and pre-Alpine limestone trout rivers, steady spring temperatures give a long build-up of olives, sometimes backed up by big burrowing mayflies where the river has the right soft bottoms, followed by strong evening caddis through summer and into early autumn; blue-winged and autumn olives plus terrestrials from meadows and woods carry fish towards the close of the trout season.