The term brown-water navy refers in its broadest sense to any naval force capable of military operations in fluvial or littoral environments, especially those carrying heavy sediment loads from soil runoff or flooding. It originated in the United States Navy during the American Civil War, when it referred to Union forces patrolling the muddy Mississippi River, and has since been used to describe the small gunboats and patrol boats commonly used in rivers, along with the larger "mother ships" that supported them. These mother ships include converted World War II-era LCMs and LSTs, among other vessels.