The Yser is a 4-letter river of Belgium, which is also known as the IJzer. It is a small river that flows through the French-speaking part of Belgium and into the North Sea. The Lys is a 3-letter river of Belgium, which is also known as the Leie. It is a tributary of the Scheldt and runs through the French-speaking part of Belgium and into the Netherlands. The Maas is a 4-letter river of Belgium, which is also known as the Meuse. It is a major river that flows through France, Belgium, and the Netherlands before emptying into the North Sea.