148: Upper Mississippi

A part of the Mississippi complex, this ecoregion includes most of Minnesota, most of western Wisconsin, part of northeastern South Dakota, most of Iowa, parts of northeastern and southeastern Missouri, most of Illinois, and a small extension into northwestern Indiana. The ecoregion is defined by the drainage area of the Mississippi and its many tributaries.

This basin includes the mainstem Mississippi River and its tributaries from its headwaters in Lake Itasca in north-central Minnesota to the confluence with the Ohio River at Cairo, IL where it flows into the lower Mississippi (Robison 1986).

Among the major tributaries of the Mississippi River are the Chippewa and Wisconsin rivers in Wisconsin, the Minnesota River in Minnesota, the Des Moines and Iowa rivers in Iowa, and the Wabash and White rivers in Illinois.

The ecoregion falls within the Central Lowlands physiographic province. Much of the area was glaciated during the Pleistocene, and thus lacks the topographic diversity of other unglaciated areas. During this time the Driftless Area remained unglaciated, displaying greater relief, including many carved river valleys (Burr & Page 1986).

This ecoregion experiences a continental climate with warm summers in the north and hot summers in the south (Köppen 1936; Kottek et al. 2006). Mean annual precipitation in the ecoregion ranges from 700 to 1150 mm (Burr & Page 1986).

There are numerous lakes, with the heaviest concentration in the northern half of the ecoregion, which was covered by a continental glacier as recently as 10,000 to 15,000 years ago. This ecoregion also contains the Driftless Area, located in the northwest corner of Illinois, southern Wisconsin, and eastern Iowa; this area is completely surrounded by once-glaciated land, but was itself never glaciated. Common in this region (as well as in ecoregions [142] and [143], and [109]) are isolated wetlands called prairie potholes, which harbor a variety of aquatic species. These wetlands are so numerous that a full catalog of species inhabiting them has never been completed.

Terrestrial habitats include mixed forest, tallgrass prairie and forest-grassland mosaic (Ricketts et al. 1999). Prior to settlement, the ecoregion was dominated by oak-hickory, beech-maple forest and bluestem prairie, with tracts of northern hardwoods, spruce-fire, pine and maple-basswood forests (Burr & Page 1986). Today, much of the land has been developed and modified for agriculture.

The Upper Mississippi ecoregion is distinguished by the presence of several remarkable species whose distributions include other ecoregions. It is the northernmost habitat for the largest of the gars, the alligator gar (Atractosteus spatula), and the paddlefish (Polyodon spathula). Also common in these waters are predatory fishes such as northern pike (Esox lucius) and muskellunge (Esox masquinongy).

Despite its diversity, the Upper Mississippi contains no endemic fish since the ecoregion was glaciated as recently as 10,000 years ago. Rare or near-endemic species of fish within the bounds of the region include the greater redhorse (Moxostoma valenciennesi), the silver lamprey (Ichthyomyzon unicuspis), and the pugnose shiner (Notropis anogenus).

The ecoregion harbors a rich diversity of freshwater mussels, crayfish and other aquatic invertebrates, including one endemic crayfish and one endemic unionid mussel. Some of the herpetofauna that inhabit the mainstem of the Mississippi River further south extend as far north as this ecoregion, and are possibly invading northward from the Mississippi refugium. Salamanders such as the lesser siren (Siren intermedia) have ranges that extend north of Tennessee only along the Mississippi. The Upper Mississippi River Basin is also an important flyway for 60 percent of all North American bird species (UMRCC 2000).

Ecoregion boundaries are taken from Abell et al. (2000) and are based on subregions defined by Maxwell et al. (1995).