Marshes are usually considered wetlands with aquatic herbaceous vegetation, either rooted or not, though some trees and shrubs are usually present as well, though not dominant. Marshes are variously known as pantano or marisma in Spanish and pântano or charco in Portuguese, though these terms are not commonly used specifically for marshes in the Amazon. Marshes are more difficult to distinguish as communities than wetland forests because they often fringe or grow interlaced with them on floodplains or along riverbanks and floodplain lakes. Nevertheless, their presence can be detected with satellite imagery and major areas of their development can be quantified during high and low water periods. For management purposes, marshes will probably have to be managed together with wetland forest because of their close proximity.
In the Amazon, marshes are most common on the whitewater floodplains where nutrients are sufficient to support large expanses of floating meadows that can consist of rooted and free-floating herbaceous plants. However, they are also common in savanna areas that are inundated mostly by relatively nutrient-poor local rainwater, but have sufficient nutrients from non-alluvial soils. Some blackwater and clearwater rivers also support marshes, though these are less developed and less productive than those of the whitewater rivers. During the floods, some marsh grasses on whitewater floodplains can grow 20-30 cm a day to accompany rising water levels. After reaching 4-6 meters, the grasses break loose from the soil and begin to float. During the dry season freshwater marshes greatly contract though many herbaceous species survive the dry phase rooted in the soil. The floating meadows found in whitewater floodplain marshes are among the most productive aquatic habitats in the Amazon Basin. They are also biologically important for many vertebrate and invertebrate animal species. For example, they are nurseries for many migratory fish species.
Where floodplains have been deforested, light conditions become sufficient for herbaceous plant communities to expand. This is especially evident on the Amazon River floodplain downriver from its confluence with the Negro River, but has probably also occurred in what are now savannas, such as in eastern Bolivia where major wetland modification took place even in Pre-Colombian times.
Outside of floodplains, extensive marshes exist on savannas that become inundated during the rainy season by local precipitation. Although dominated by herbaceous vegetation, they almost always have scattered shrubs and often trees, especially when bordering streams. The most notable are the Llanos de Moxos of Eastern Bolivia, Bananal Island between the Tocantins and Araguaia rivers and the Roraima savannas of the Branco Basin.