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Lower St. Croix and Vermillion Valleys

↑About the Lower St. Croix and Vermillion Valleys

The Lower St. Croix and Vermillion Valleys, referred to in some documents as the Prairie Pothole Region, is a small region roughly split in half between eastern Minnesota and western Wisconsin, located slightly east to south of the Twin Cities, Minneapolis and St. Paul. This region includes the portion of the Mississippi river just upstream from the Driftless area, and is thus the last glaciated region that the river flows through before entering the driftless area. In some documents, this region is grouped in with a broader region, called the Southern Oak Plains that covers this region but also extends much farther south.

The terrain here includes nearly level floodplains and outwash plains on uplands, to rolling terrain on the higher ridges, and some steeper ravines, bluffs, and dissected terrain along the rivers and larger channels that drain into them. The bedrock consists mostly of dolomite, and in most areas this is covered with glacial till, sandy outwash, and fine wind-blown loess. The most common configuration is a thin layer of loess with coarser, sandy-to-gravely drift underneath. Soils vary considerably by site, but tend to be nutrient-rich and ideal for Western agriculture. Much of the region is covered in nutrient-rich mollisols that formed under prairies, derived from loess, and are mostly well-drained loams, although in some places the soils are coarser sands. Alfisols, also nutrient-rich, are found mostly on steeper slopes of the ravines, and formed under forests. Floodplains feature rich alluvial soils.

The climate is humid and continental, near the border of a hot-summer climate with the cooler, hemiboreal climate. There are four seasons, but temperatures change quickly in spring and fall. Precipitation is seasonal, mostly falling in summer, but this seasonality is largely driven by temperature, and the climate tends to be humid year-round. Due to the interior location, largely-flat surroundings, and distance from large bodies of water, rapid changes in weather, sometimes severe, are possible at any time of year.

Originally this region was mostly covered in tallgrass prairie, with small areas of mixed hardwoods, with sugar maple (Acer saccharum) and American basswood (Tilia americana) the dominant trees, mostly on the steeper terrain. There was likely some bur oak (Quercus macrocarpa) savanna in areas transitional between prairie and forest, but overall there was much less savanna here than areas to the south. The tallgrass prairie was maintained by fire. Native Americans may have increased the portion of prarie here beyond what would naturally occur, through their use of fire.

Nowadays, roughly half the land area here has been cultivated as corn and soybeans; a smaller amount is used as pastureland, and an even smaller amount, mostly on the steepest terrain, is left as forest. Fire suppression has also resulted in some formerly prairie sites converting to forest. The Willow River, the largest tributary in this region flowing into the St. Croix river, has been dammed at the Willow River Saint Croix Dam, near Hudson and North Hudson, creating what is now called Lake Mallalieu; this region has had a history of various dams, sometimes being damaged in severe rain events and resulting in flooding of areas where people had built.

Although most of this area is rural, in recent years the Twin Cities metro area has expanded and some of its outermost suburbs are now found here, mostly in the southern portion of this region, but also to a lesser degree in the east. The suburbs here are largely low-density, car-oriented exurbs built in recent years, and have a large footprint relative to their population. Outside these suburbs, the largest cities here are Hastings, MI and Hudson, WI, and a number of small towns, as well as some formerly small towns have grown into exurbs of the Twin Cities, such as Farmington, MN.

At the limit of glaciation to the east and south, this region meets the Driftless Area, in most places the Blufflands and Coulees, except in the very south, where it instead borders the Rochester/Paleozoic Plateau Upland. To the south, this region opens up onto the much larger Eastern Iowa and Minnesota Drift Plains, an area with a greater portion of bur oak savanna than here. The southernmost portion of this region is bordered to the west by the Big Woods, and the easternmost portion is bordered to the north by the Central Wisconsin Undulating Till Plain; both of these regions were originally forested but are now largely agricultural. The central portions of this region are bordered to the northwest by the St. Croix Stagnation Moraines, a region with more hummocky terrain.

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