Elevations range from 90 m to greater than 250 m. Slope gradient is generally less than 1 in the center of the ecoregion, and 1 to 2 at the edges. The region is mantled by Quaternary alluvial deposits. Aeolian silt and sand deposits cover some areas. The Yukon River drains the ecoregion, assisted by numerous meandering and braided tributaries and side sloughs. Permafrost is present in most areas, except beneath rivers and large thaw lakes.
Thaw lakes are abundant, as are oxbow lakes.
The region was free from glaciation during the Pleistocene epoch. Soils Principal soils are Histic Pergelic Cryaquepts, Pergelic Cryaquepts, Aquic Cryochrepts, and Pergelic Cryochrepts. Most soils formed from silty alluvium and loess from the floodplains of the large rivers. On flat areas away from the main river channels, soils are poorly drained, commonly overlain by peat, and have a shallow permafrost table. Soils on the natural levees are better drained and consist of silty and sandy sediments.