Backwaters include a variety of alluvial floodplain waterbodies. Tertiary and smaller tributary channels are included in backwater areas. Depending on study needs, aquatic habitat in the UMRS can be defined according to vertical spatial categories. Within aquatic areas are the water surface, the water column, the surfaces of cover structures such as aquatic macrophytes, rocks, and woody debris, the interior of cover structures, the sediment surface, the hyporheic zone within the sediment, and the deeper sediment. Aquatic areas are spatially large areas of somewhat similar aquatic habitat defined according to geomorphic and navigational features of the river.
Habitat conditions are physical and chemical variables that occur in aquatic habitat.
Habitat types are defined as needed for the application, using aquatic area and some combination of habitat conditions. No formal classification of aquatic habitat types is proposed. Habitat type definitions can be developed according to the needs of different investigations. The main channel conveys the majority of the river discharge and in most reaches includes the navigation channel. In reaches where buoys are not used, the centerline of the navigation channel is defined by lights and daymarks on shore that pilots use for navigation.