Historically a very polluted river from the high population density and industrialisation in both Northern France and Belgium, it has seen substantial improvements in recent years, partly due to the decline of the principal industry, the spinning and weaving of flax.

The region of the Leie (between Deinze and Ghent) was known as a favourite place for numerous painters in the first half of the 20th century.

It flows generally northeast through the following departments of France, provinces of Belgium and towns and municipalities: The main tributaries of the Leie are, from source to mouth: Laquette, Clarence, Lawe, Deûle, Gaverbeek [nl], Heulebeek [fr; nl; vls], and Mandel.

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 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.