The presence of a histic epipedon in a Fluvaquent would be a criterion at the series category of Soil Taxonomy provided the epipedon is at the soil surface and thus is not a buried genetic horizon. Mineral soils can be differentiated from one another as separate series based on the presence of a histic epipedon. The subgroup that a Fluvaquent classifies into depends on the soil properties present and the keying sequence for the subgroups. A Fluvaquent with a histic epipedon might also have an underlying mineral epipedon such as an ochric epipedon. Such a mineral epipedon might be darkened by humus but it also might be too thin to meet the thickness requirements for an umbric or mollic epipedon. These soils have the potential to classify in either the humaqueptic or mollic subgroup and they would also have the histic epipedon as an additional diagnostic horizon for series separation.
Histic epipedons that are buried would be classification criteria for the thapto-histic subgroup provided that sulfidic materials or vertic intergrade properties are not also present in the soil.
Endoaquents follow a similar classification scenario to the Fluvaquents except that there is not a thapto-histic subgroup available in this great group.
