AN INTEGRATED RECORD FOR LATE HOLOCENE CLIMATE IN THE NORTHERN U.S. WESTERN INTERIOR FROM STRATA OF THE LOWER MISSOURI RIVER: EVIDENCE FOR A REGIONAL CLIMATE SHIFT AT 3500 B.P

The Missouri River drains a 10-state region encompassing the whole of the northern U.S. Western Interior. Numerous individual studies have gained insights into Holocene climatic evolution of this large region by study of dispersed lakes and tributary drainages. Most of these studies argue locally for a shift from a generally dryer middle Holocene climate toward a wetter late Holocene climate. The Missouri River will integrate these local climate signals as it drains this large region. Localized climate signals will thus tend to filter out as tributaries merge. Only the additive/regional signals should remain as the river enters the lower-valley reaches.

This study examines a 50 km alluvial reach of the lower Missouri River floodplain in order to test for evidence of major Holocene climate shifts that altered sediment and water supply throughout the drainage basin. The study reach extends for approximately five channel wave lengths and begins 50 km east of Kansas City, MO. We mapped surficial alluvium allostratigraphically over this reach in order to determine past pattern and position of channel loops and to seek evidence for past terracing events. Chronology of loops was then assessed using cross-cutting relationships and OSL dating.

This study reveals that the Missouri River has changed form and has experienced terracing during the middle to late Holocene transition. The middle Holocene river maintained a single-channel high-sinuosity form that initiated by at least 5.0 to 4.5 k.b.p. The river somewhat abruptly established a low-sinuosity form with at least 1/3 island-braided reaches at approximately 3.5 k.b.p. By about 1.5 k.b.p., this transitional form had gradually given way to a dominantly island-braided form with even lower sinuosity. This transitional period included a 3 m incision event between 3.0 and 2.5 k.b.p.

These channel shifts are too consistent in timing and location to record local channel anomalies, and are too pervasive to record local tectonic control. These trends thus appear to record channel response to a major regional change in climate trend within the drainage basin that began at about 3.5 k.b.p. The cause of this trend is uncertain, but the timing is consistent with previous studies that suggest major climate change at this time.