Composed of broad basins and flatlands between large rivers in interior Alaska, the Yukon-Kuskokwim bottomlands are dominated by lakes, streams, and other riparian features such as sloughs and wetlands. The ground is generally level, but there are a few rolling hills and elevations range from 120-650 meters. Soils are derived from alluvial materials on floodplains and wind-blown loess on hills.
This is a relatively warm and dry area, especially in the summer and thus wildfires are common and significant events. River flooding is also common, especially in spring when snow in the Alaska Range is melting. Vast expanses of open black spruce (Picea mariana) cover this ecoregion, interspersed with white spruce (Picea glauca) and poplar (Populus spp.) forests along rivers, and thickets of willow (Salix spp.) and alder (Alnus spp.) in floodplains. Wet sedge meadows are also common and associated with lakes. Permafrost is frequent.