Tributaries to the Mississippi River record paleowater levels of the main channel. When the river stage rises, water backs up into the tributaries and deposits slackwater sediment. If river levels are high for an extended time, the tributaries become temporary lakes and unoxidized lacustrine sediment accumulates.

Cores from tributaries along the Western Lowlands and the Eastern Lowlands of the northern Mississippi Alluvial Valley show that many of these valleys were inundated by floodwater associated with late Wisconsin deglaciation for extended periods. The floodwater inundates the larger Ozark tributaries of southeastern Missouri at least 15 km upvalley. Accumulation of bedded gray and red clay, gray silty clay, and gray very fine sand extends to 18 m depth. This accumulation of sediment buried the valley bottom topography, including a floodplain and at least one terrace, in several streams. Paleowater levels were continuously high during the accumulation of this slackwater sediment at most locations because there is no evidence of oxidation, plant roots, or burrows that might occur if the water level receded and the sediment became exposed. In addition, the sediment is very poorly consolidated because it has never been dewatered.

Further south in northeastern Arkansas, an Ozark tributary is also partly filled by slackwater sediment. However, the sediment fill appears to be less than that to the north, extending only to 7 m depth. In contrast, tributaries on the eastern side of Crowleys Ridge (the divide between the Western and Eastern Lowlands) were not permanently inundated and have no slackwater fill. This thinner to absent fill in tributaries to the south suggests that paleowater levels in the Mississippi Valley became relatively lower or sediment load was reduced downstream.