The river rises at Gouy-en-Ternois and passes Frévent, Hesdin, and Montreuil-sur-Mer before leaving the chalk to flow to the coast between Étaples and Le Touquet-Paris-Plage.

The lie of the land means there is no notable tributary from the south until the Grande Tringue, which flows from marshland into the small, dredged estuary.

The unhealthy aspect of marshland means much has been done over the centuries to drain the land efficiently, which has brought about the partial destruction of its original character.

Further developments in the 18th century saw permanent enclosures with animals being fenced-in and property boundaries being created by the planting of hedges and the digging of ditches, contributing to organized and cooperative farming methods.

During the 19th century, technical progress (for example, the replacement of windmills by steam engines) led to further improvements in drainage, contributing to the drying-up of the valley downstream as far as Hesdin.

The mobilisation of the local inhabitants, various associations and many scientists, after many public enquiries and heated arguments, stopped the plans and left the estuary to nature.