Optical Dating of the St. Croix River Spillways, Wisconsin and Minnesota

The Upper St. Croix River valley in northern Wisconsin and Minnesota is a deeply incised drainage network, which formed during the final retreat of the Laurentide ice sheet. The St. Croix River valley acted as a spillway numerous times for Glacial Lake Duluth, a proglacial lake which formed in the Superior basin, and possibly other glacial lakes in northern Minnesota and the Upper Peninsula of Michigan. Today, the modern river level is incised to a depth of greater than 30 m in many areas, with valleys consisting of numerous terraced spillways. A ~95 km stretch of the St Croix River Valley to the west of Grantsburg Wisconsin shows evidence for a minimum of three major overflow events based on its terrace or spillway levels. Locally, some of the abandoned spillways are greater than 3 km in width, suggesting formation resulting from the drainage of one or more glacial lakes to the north. The formation and drainage timing of regional glacial lakes are unknown, thus the timing for the incision of the spillways and the St. Croix River are not well understood. This study uses optical dating techniques to better constrain the timing for the incision of the spillways and the modern St. Croix River channel. Optical age estimates from older St. Croix River alluvium suggests a maximum age for incision of the oldest spillway level to be ~11 ka. Further optical ages obtained from alluvium and fluvial features preserved in the spillways will help to better understand the timing and origin of events which formed the Upper St. Croix River valley.