The Natural Divisions of Missouri
The Natural Divisions of Missouri (fig. 7) divide the state into six major regions about which generalizations regarding bird distribution and relative abundance can be made: Glaciated Plains, Ozark Border, Ozark, Osage Plains, Big Rivers and Mississippi Lowlands (Thom and Wilson 1980). Geologic history, soils, bedrock geology, topography, plant and animal distribution, pre-settlement vegetation and other natural factors are all used to define division boundaries. Each division represents part of a larger physiographic region which extends beyond Missouri's borders. Because each division contains a variety of landscapes and natural communities which add considerable diversity, each is divided into natural sections.
Figure 7. Missouri's Natural Divisions & Sections
Western Glaciated Plains
Eastern Glaciated Plains
St. Francois Mts.
Glaciated Plains Natural Division
Roughly encompassing the northern one-third of the state, the Glaciated Plains Natural Division is characterized by soils and topography that resulted from the influence of Pleistocene glaciation about 400,000 years ago. Soils are formed from loess and glacial till or from alluvium. The topography is younger than that of the unglaciated portions of the state, although much of this division is moderately dissected. Upland and bottomland deciduous forest and prairie were the main pre-settlement vegetation, with prairie comprising about 45 percent of this division.
The Western and Central natural sections of this division are characterized by loess-dominated topography and soils. The Western Natural Section is more highly dissected and has the driest climate in the state. The Central Natural Section contains soils derived from glacial till and is more moderately dissected. Today, agriculture dominates the landscape with large fields of corn, soybeans and pasture. Woodlands suitable for forest interior breeding birds are rare except at a few of the region's state parks and conservation areas.
The Eastern Natural Section includes a region of flat, claypan soils typified by the Audrain plains, where corn and soybean production predominates today. This natural section also includes rugged river breaks such as those along Salt River. Mark Twain Lake, an impoundment of the Salt River, is a major feature of this section. Farther to the east, the Lincoln Hills Natural Section extends along the Mississippi River in Lincoln, Pike, Ralls and Marion counties. The Lincoln Hills, which may have partially escaped glaciation, are distinguished from the rest of the division by bedrock geology, caves, and steep, heavily- forested areas.
Ozark Border Natural Division
The Ozark Border Natural Division comprises about 13 percent of the state. It extends along both sides of the lower Missouri River and the lower Mississippi River to the Mississippi Lowlands Natural Division. It includes rugged river hills with deep, relatively productive soils and a few isolated rolling plains. Although upland deciduous forest was the main pre-settlement vegetation, glade, marsh, prairie and bottomland forest communities were also present. Prairie accounted for less than 10 percent of the pre-settlement vegetation. Many of the soils are derived from loess and are relatively productive. Today, rolling pastures, shrubby, fragmented forests and forested bottomlands characterize this division.
Ozark Natural Division
The Ozark Natural Division is a large, unglaciated region of greater relief and elevation than the surrounding areas. This division comprises almost 40 percent of the state. It is characterized by thin, often stony, residual soils. Topography is often very steep. Caves, springs, bluffs, and high-gradient, clear-flowing streams with entrenched meanders are characteristic features. Deciduous, pine-oak and pine forests formed the predominant vegetation in pre-settlement times. Glades, some of them extensive, commonly occur where bedrock surfaces. Bottomland deciduous forests are common along many of the streams. The great age and physiographic diversity of the Ozarks make it the region of greatest species diversity in Missouri. This division is divided into six sections.
The Salem Plateau Natural Section is a broad, gentle plain that was originally forested to a great extent but is now characterized by open pastureland and scattered trees. Bottomland deciduous forests remain along many streams.
The Lower Ozarks Natural Section is richly forested and characterized by springs, caves, sinkholes, calcareous wet meadows, glades, clear, high-gradient streams and steep-sided hills with narrow, chert-covered ridges. Short-leaf pine (Pinus echinata) is also characteristic. Streams flow generally southward and include the St. Francis, Black, Current and Eleven Point rivers.
The St. Francois Mountains Natural Section is the area of greatest relief and elevation in Missouri. There are few springs, caves or other karst features in this natural section. The pre-settlement vegetation of pine and mixed pine-oak still predominates.
The White River and Elk River natural sections are both characterized by steep terrain and deciduous forest mixed with some pine. Glades are common, especially in the White River drainage. Construction of reservoirs (Table Rock, Taneycomo and Bull Shoals lakes), has created habitat for a few wetland-associated breeding birds while inundating habitat for bottomland forest species.
The Springfield Plateau Natural Section is physiographically the most distinct in the Ozark Natural Division. Historically, its landform was not highly dissected and its topography, soils, and pre-settlement vegetation were characterized by a mosaic of Osage Plains prairies grading into Ozarks forests. Prairie once occupied about 29 percent of this section, which today is less than 1 percent prairie. Today, this natural section is characterized by fragmented forests, pasture and early successional shrub-scrub habitats.
Osage Plains Natural Division
The Osage Plains Natural Division, which occupies about 8 percent of the state, is an unglaciated region in central western Missouri with an open, grassland aspect and gently rolling topography. Compared to the prairies of the Glaciated Plains, the upland prairie of this natural division has a greater proportion of southwestern plants and animals, fewer northern species, and a greater diversity in stream-side woody vegetation. More than 70 percent of the Osage Plains was prairie in pre-settlement times. Savanna, upland and bottomland deciduous forest and marsh also occurred. Streams commonly had shallow valleys and broad floodplains with many sloughs and marshes.
Today, much of the Osage Plains Natural Division is used for agriculture. Much of the area is pastureland with row cropland dominating in only a few areas. With the absence of frequent prairie fires, many savannas have succeeded to scrub-land and second-growth forests. The upper reaches of Truman Reservoir provide some wetland habitat for breeding birds at the Schell-Osage, Four Rivers and Montrose conservation areas. The highest quality prairie habitat remaining in the Osage Plains is on public prairies, including those on the Taberville and Paint Brush Prairie conservation areas and at Prairie State Park.
Big Rivers Natural Division
The Big Rivers Natural Division, comprising about 5 percent of the state, includes the floodplains and terraces of the largest rivers, primarily the Missouri and Mississippi, but also the lower Grand and the lower Des Moines rivers. Soils are mostly alluvial, deep and productive. Pre-settlement natural features included mesic to wet prairie, bottomland and upland forests, marshes, sloughs, islands, sandbars, mud flats, oxbow ponds and rivers. In pre-settlement times, and until extensive channel modification began in the early 1900s, the Missouri River was a braided stream with many chutes, sloughs, islands and seasonal wetlands.
Between 1879 and 1972, about 50 percent of the original surface area of the Missouri River was lost, backwater habitat was eliminated and the main channel was deepened and narrowed. Most of the original natural communities are no longer present. The Mississippi River, although less changed than the Missouri, has been modified and the addition of locks and dams has converted portions of the Mississippi into a series of large pools.
Today, this is the second-most intensively farmed natural division of the state, with corn, soybeans and wheat the most common crops. Wildlife habitat is preserved within a number of managed wetland areas, including Squaw Creek, Swan Lake and Mark Twain national wildlife refuges, and Bob Brown, Ted Shanks and Fountain Grove conservation areas.
Mississippi Lowlands Natural Division
The Mississippi Lowlands Natural Division comprises about 5 percent of the state. The Lowlands Natural Section, one of two natural sections in this natural division, is primarily composed of flat, alluvial plain and low terraces at the head of the Mississippi Embayment. This division has the highest average precipitation and temperature in Missouri. Relief is slight, with much of the division less than 100 meters above sea level. Historically, much of this division was bald cypress (Taxodium distichum) and tupelo gum swamp (Nyssa aquatica) forest, mixed deciduous bottomland forest, and low upland deciduous forest. Clearing and draining began in the early 1900s. Conversion to agriculture has been almost total and today only small remnants of natural forest and swamp remain.
The deep, alluvial soils, and water available for irrigation have made this natural division the most heavily farmed in the state. Today, much of the natural division is composed of large fields of soybeans, wheat, corn, rice and cotton. Wildlife habitat is provided by several public wetlands including Mingo National Wildlife Refuge and Duck Creek, Otter Slough, Coon Island and Ten Mile Pond conservation areas. A few large tracts of forest remain along the Mississippi River, including Ben Cash and Donaldson Point conservation areas and Big Oak Tree State Park. Drainage ditches and isolated forest fragments provide limited habitat for wetland and woodland species. Certain agricultural practices, including the flooding of rice fields, simulate natural habitat for some wetland-associated breeding birds.
Crowley's Ridge Natural Section is this division's most prominent topographic feature, rising above the surrounding lowlands in a disjunct series of forested, low hills. This section of the Mississippi Lowlands resembles the forest- and pasture-dominated Ozarks and supports many of the bird species characteristic of the Ozark and Ozark Border natural divisions.