Environmental Description:  This association occurs in the interior valleys of western Oregon. Proximity to the Pacific coast is modified by the coastal mountain ranges to the west. Winters are mild and wet, but summers are hot and dry. Roughly 80% of the annual precipitation (average of 100 cm) falls during October through May, primarily as rain. The persistent, low-intensity rains result in soil saturation and flooding of low-lying valley bottoms. These valleys are flat-bottomed and characterized geologically as incised valleys filled by deep alluvial materials, over which silts and clays have been deposited. Topographically, there are broad, alluvial flats separated by groups of low hills. Elevations range from 50 to 140 m (160-450 feet), and the low relief has resulted in the rivers being slow-flowing and meandering. This association occurs in old valley bottom river oxbows or bottomland swamp and lake remnants. The soils are derived from heavy clays overlain by thin silt deposits, and horizonation is typically depositional rather than developmental. There is often an abrupt transition from silt loam textures of the surface horizon to clay in the deeper horizons, with a resulting high water-holding capacity and poor drainage. The sites are seasonally flooded, but dry out for up to 4 months, typically from late July to early October.