AN EARLY HOLOCENE CATASTROPHIC FLOOD ORIGIN FOR THE ST. CROIX RIVER VALLEY IN THE UPPER MISSISSIPPI RIVER BASIN

Geomorphic, sedimentologic, stratigraphic and radiocarbon evidence suggests that the St. Croix Valley as it exits today was carved by the last deglacial catastrophic flood to course through the upper Mississippi River valley.

The St. Croix River has very low sinuosity and numerous islands in the reach north of Stillwater. South of Stillwater, the river turns into a riverine lake, Lake St. Croix, presumably in response to hydraulic damming by the Mississippi River. Catastrophic flood landforms are the highest geomorphic surfaces inset below the surrounding uplands, but they are also some of the lowest in the valley. The most deeply scoured inner channel formed by late deglacial catastrophic flooding confines the St. Croix River, its narrow floodplain, and Lake St. Croix. Other catastrophic flood landforms include bars, marginal channels, erosional residuals and erosional benches. Associated sediment assemblages overlie bedrock or glacial drift, including rhythmically bedded and laminated reddish brown clay and silt suggestive of a pre-existing proglacial lake.

Multiple radiocarbon ages from the base of thick peat that fills the highest marginal flood channels indicate that the catastrophic flood responsible for cutting the St. Croix Valley is very early Holocene in age (<10.1, >9.4 ka B.P.). Basal organic muds that post-date the flood were collected and dated from Lake St. Croix, the lowest surface in the valley, and are considered by Lund and Banerjee (1985) to be about 9.6 ka B.P. in age. These age limits coincide with a marker bed found in the upper and lower Mississippi Valley that dates between 9.9 and 9.7 ka B.P.; the bed consists of a mineralogically distinct reddish brown clay with a Lake Superior Basin source (Hajic and Bettis, 1997). The flood was triggered by events in the Lake Superior basin during the Marquette advance of the Lake Superior Lobe (ca. 9.95 – 9.70 ka B.P.; Phase D of Clayton, 1983). Geomorphic evidence suggests that both the Kettle and Brule rivers carried floodwaters simultaneously to the St. Croix. This flood probably formed the principal geomorphic features of the St. Croix Valley. The marker bed in the Mississippi Valley was deposited as floodwaters waned. Furthermore, it is expected that the youngest glacial meltwater isotopic excursion from the Gulf of Mexico will date to this flood event.