The Leie/Lys was a commercial navigation from the Middle Ages, but it was the river's devastating floods rather than navigation improvements which justified major works and meander cut-offs started around 1670. The 9 meter difference in elevation between Aire-sur-la-Lys and the border was gradually overcome by six locks and weirs, completed in 1780. The river carried a heavy traffic in grain and linen through to Ghent and Antwerp. The navigation was leased out to a company around 1825, and the locks upgraded to 5.20 m wide, for a draught of 1.60 m. In December 1899, more than 40 schoolchildren skating on the frozen river at
Frelinghien fell through the ice and drowned. The river was given its present depth by the
Freycinet programme. The section below Armentières was enlarged to class III from 1930, and the border section was improved to class Va starting from 1960. This section is part of the EU's priority project 30 for the Seine-Escaut waterway. The upstream section, by contrast, is used almost exclusively by recreational craft, and is at present heavily silted. == References ==