Comentário: "Large" is ambiguous. For details on "Large rivers" (e.g. the Mekong river) see http://worldwildlife.org/biomes/large-river-ecosystems This class will be replaced with a less ambiguous class.
Definição: Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. [ https://en.wikipedia.org/wiki/Silt ]
Definição: Gravel is an environmental material which is composed of pieces of rock that are at least two millimeters (2mm) in its largest dimension and no more than 75 millimeters.
