The riverine system is formed and constantly modified by fluvial (channel stream flow) and other hydrologic processes. These processes influence wetland occurrence and extent. Because of the fluvial and groundwater interaction with landform, soil, and vegetation, our discussion will start with these dynamic processes. The discussion of features within the riverine system focuses on the floodplain, which is their most common and often defining feature. Wetlands and hydric soils also occur at the interface of uplands and river valley terraces, at the headwaters of the riverine system as groundwater seeps (see Chapters 3 and 9), and they are associated with oxbows and related features.