After the marshland between the river Seine and its slower 'dead arm' to its north was filled in from around the 10th century, Paris's cultural centre began to move to the Right Bank. In 1137, a new city marketplace (today's Les Halles) replaced the two smaller ones on the Île de la Cité and Place de la Grève (Place de l'Hôtel de Ville). The latter location housed the headquarters of Paris's river trade corporation, an organisation that later became, unofficially (although formally in later years), Paris's first municipal government.