According to the
United States Census Bureau, the county has a total area of , of which is land and (2.9%) is water. Colloquially, St. Louis County is often divided into Mid, North, West, and South sections. North County lies north of
Interstate 70, West County lies west of
Interstate 270, South County lies south of
Interstate 44 and Mid County lies in the middle of the three main bordering highways (I-70, I-270 & I-44) and the St. Louis county-city line.
Natural boundaries The
Missouri River forms the northern border with
St. Charles County, exclusive of a few areas where the river has changed its course. The
Meramec River forms most of its southern border with
Jefferson County. To the east is the City of
St. Louis and the
Mississippi River. The western boundary with
Franklin County is the north–south line where the distance between the Meramec and Missouri rivers is the shortest, bisecting the city of
Pacific, roughly two blocks east of Hwy OO/F (First street).
Topography The foothills of the Ozark Mountains begin in southwestern St. Louis County, with most of the rest of the county being a fairly level plateau. This western part of the county is the least developed, due to rugged topography. Bluffs along the Mississippi in the south of the county rise about 200–300 feet above the river. A major floodplain area is the
Chesterfield Valley, in the western part of the county, along the Missouri River. It was formerly called "Gumbo Flats" after its rich, dark soil; it was submerged by at least ten feet of water during the
Great Flood of 1993. The Corps of Engineers constructed a higher levee, and the county has permitted construction in the floodplain. The Columbia Bottom is a floodplain in the northeast of the county at the
confluence of the Mississippi and Missouri rivers; this is preserved as a conservation area open to the public. The Missouri Bottom area between the two other floodplains had been developed for agriculture, but it is being increasingly developed for residential, business, and industry. The
River des Peres drains the interior of the county before flowing into constructed underground channels into the City of St. Louis. It was allowed to resurface, where it forms the boundary between southern portions of St. Louis City and St. Louis County. Other streams include
Coldwater Creek,
Bonhomme Creek, and Creve Coeur Creek, flowing into the
Missouri River;
Keifer Creek, Fishpot Creek, and Grand Glaize Creek, flowing into the
Meramec River; Deer Creek and
Gravois Creek, flowing into the
River des Peres; and
Maline Creek, flowing into the
Mississippi River. The highest elevation is .
Geology The bedrock is mainly
limestone and
dolomite, and much of the county near the rivers is
karst terrain, with numerous caves, sinkholes, and springs. No
igneous or
metamorphic rock is exposed on the surface. A major outcropping of the
St. Peter Sandstone formation, a fine white sandstone used for making clear glass, is mined in the southwest corner of the county in
Pacific. Brick clay mining was once a major industry in the county. The Charbonier Bluff along the Missouri River is an outcropping of coal and was used a fueling station for
steamboats. The "St. Louis Anticline", an underground formation, has small petroleum deposits in the north part of the county.
Flora and fauna Before European settlement, the area was prairie and open parklike forest, maintained by
Native Americans via burning. Trees are mainly
oak,
maple, and
hickory, similar to the forests of the
Ozarks; common understory trees include
eastern redbud,
serviceberry, and
flowering dogwood.
Riparian areas are heavily forested with mainly
American sycamore. By the 1920s most of the timber in the county was harvested. Since that time, large parks and undeveloped areas in the western and southern parts of the county have grown dense forest cover. Old pastures are usually colonized with
eastern red cedar. Most of the residential area of the county is planted with large native shade trees. In autumn, the changing color of the trees is notable. St. Louis County has the most recorded native species of plants in the state, but this is probably due to the intensive botanical research done in the area. Most species here are typical of the Eastern Woodland, but some southern species are found in swampland, and typical northern species survive in sheltered
hollows. Invasive species, most notably
Japanese honeysuckle, are common in some homesteads converted to parks; these are actively removed. Large
mammals include growing populations of
whitetail deer and
coyotes, which are becoming increasingly urbanized.
Eastern gray squirrel,
cottontail rabbit, and other rodents are abundant, as well as
opossum,
beaver,
muskrat,
raccoon, and
skunk. Large bird species include
wild turkey,
Canada goose,
mallard duck, various
raptors like the
turkey vulture and
red-tailed hawk, as well as
shorebirds, including the
great egret and
great blue heron. Winter populations of
bald eagles are found by the
Mississippi River around the
Chain of Rocks Bridge. The county is on the
Mississippi Flyway, used by migrating birds, and has a large variety of small bird species, common to the eastern U.S. The
Eurasian tree sparrow, an introduced species, is limited in North America to the counties surrounding St. Louis. Frogs are commonly found in the springtime, especially after extensive wet periods. Common species include
American toad and species of chorus frogs, commonly called "
spring peepers", that are found in nearly every pond. Some years have outbreaks of
cicadas or
ladybugs.
Mosquitos and
houseflies are common
insect nuisances; because of this, windows are nearly universally fitted with
screens, and "screened-in"
porches are common in homes of the area. Populations of
honeybees have sharply declined in recent years, and numerous species of
pollinator insects have filled their ecological niche.
Climate St. Louis County has a mix of a humid subtropical climate and a humid continental climate, with neither large mountains nor large bodies of water to moderate its temperature. The area is affected by both cold Canadian Arctic air, and also hot, humid air from the
Gulf of Mexico. The county has four distinct seasons. Spring is the wettest season and produces erratic severe weather ranging from
tornadoes to
winter storms. Summers are hot, and the humidity can cause the
heat index to rise to temperatures above . Fall is mild, with lower humidity and can produce intermittent bouts of heavy rainfall with the first
snow flurries usually forming in late November. Winters are cool to cold with periodic snow and temperatures often below freezing. Winter storm systems, such as
Alberta clippers, can bring days of heavy
freezing rain,
ice pellets, and
snowfall. The average annual temperature for the years 1971–2000, recorded at
Lambert–St. Louis International Airport, is , and average precipitation is . The average high temperature in July is , and the average low temperature in January is , although these values are often exceeded. Temperatures of or below occur three days per year on average. The highest temperature ever recorded in St. Louis was , on July 14, 1954, while the lowest temperature on record is , on January 29, 1873. Winter is the driest season, averaging about 6 inches of total precipitation. Springtime (March through May), is typically the wettest season, with under 10.5 inches. Dry spells of one or two weeks' duration are common during the growing seasons.
Thunderstorms can be expected on 40 to 50 days per year. A few of them will be severe with locally destructive winds and large
hail, and occasionally accompanied by tornadoes. A period of unseasonably warm weather late in Autumn known as
Indian summer is common—
roses will still be in bloom as late as November or early December in some years.
Other geography The largest natural lake in the county is
Creve Coeur Lake. It was originally an oxbow of the nearby Missouri River and is now the centerpiece of a popular county park. Manchester Road (
Route 100) follows an ancient path westward out of St. Louis, following the boundary between the
Missouri and
Meramec watersheds. It is one of only two routes to leave the county without crossing any rivers (the other being State Highway T). The Sinks is a
karst area in the far northern part of the county, with numerous
sinkholes. ==Demographics==