Mouse (Fr. pron. nearly Mehz; Dutch Maas), a river of France and Belgium, rises in the French dep. of Haute-Marne, and flows 500 miles N., then E., again N., and W. past Verdun, Sedan, Namur, Liege, and Maestricht, until it joins the Waal, one of the mouths of the Rhine, from the left opposite Gorkum. The united streams take the name of the Maas, which soon divides again. The southern branch passes through the Bies-bosch and Hollandsche Diep, and, again dividing, reaches the sea in two wide estuaries, Haringvliet and De Krammer. The northern branch, called the Merwede as far as Dordrecht and to the west of that town the Old Maas, likewise reaches the sea in two channels, the Old and the New Maas. On this last stands Rotterdam (q.v.). The river is navigable from Verdun. Its principal affluents are the Sambre and Ourthe on the right.