Sand Lake National Wildlife Refuge contains wetlands of exceptional importance that provide clean water, food, flood control, recreation, and vital feeding and nesting habitat for thousands of migratory birds, including shorebirds, waterfowl, wading birds, and colonial nesting birds. In 1998, the refuge was designated a Wetland of International Importance by the Ramsar Convention on Wetlands, an international treaty signed in Ramsar, Iran, in 1971 to promote the conservation of global wetlands and recognize sites that significantly contribute to wetland protection and their values. By 2003, more than 1,300 such sites had been designated worldwide, including 19 in the United States. Sand Lake earned this distinction especially for its importance to colonial nesting birds, with large colonies of gulls, terns, grebes, herons, egrets, and other birds nesting in emergent vegetation over the water, and the wetlands once supported the largest nesting colony of Franklin's gulls in the world, with over 150,000 nesting pairs.