The city has special sites for birdwatching of migratory species, including Tower Grove Park. St. Louis experiences thunderstorms 48 days a year on average. Especially in the spring, these storms can often be severe, with high winds, large hail and tornadoes. Lying within the hotbed of Tornado Alley, St. Louis is one of the most frequently tornado-struck metropolitan areas in the U.S. and has an extensive history of damaging tornadoes. Severe flooding, such as the Great Flood of 1993, may occur in spring and summer; the melting of thick snow cover upstream on the Missouri or Mississippi Rivers can contribute to springtime flooding. The city’s eastern boundary is the Mississippi River, which separates Missouri from Illinois.
According to the 2010 United States Census, St. Louis had 319,294 people living in 142,057 households, of which 67,488 households were families. The population density was 5,158.2 people per square mile (1,990.6/km2).
The city is built on bluffs and terraces that rise 100–200 feet above the western banks of the Mississippi River, in the Midwestern United States just south of the Missouri-Mississippi confluence. Much of the area is a fertile and gently rolling prairie that features low hills and broad, shallow valleys. Both the Mississippi River and the Missouri River have cut large valleys with wide flood plains. The city is divided into 79 government-designated neighborhoods. The neighborhood divisions have no legal standing, although some neighborhood associations administer grants or hold veto power over historic-district development. Several examples of religious structures are extant from the pre-Civil War period, and most reflect the common residential styles of the time.
About 24% of the population was 19 or younger, 9% were 20 to 24, 31% were 25 to 44, 25% were 45 to 64, and 11% were 65 or older. Winter populations of bald eagles are found along the Mississippi River around the Chain of Rocks Bridge. The city is seo test tool on the Mississippi Flyway, used by migrating birds, and has a large variety of small bird species, common to the eastern US. The Eurasian tree sparrow, an introduced species, is limited in North America to the counties surrounding St. Louis.
The Missouri River forms the northern line of St. Louis County, except for a few areas where the river has changed its course. Near the southern boundary of the city of St. Louis (separating it from St. Louis County) is the River des Peres, practically the only river or stream within the city limits that is not entirely underground. Most of River des Peres was confined to a channel or put underground in the 1920s and early 1930s. The lower section of the river was the site of some of the worst flooding of the Great Flood of 1993. Limestone and dolomite of the Mississippian epoch underlie the area, and parts of the city are karst in nature. This is particularly true of the area south of downtown, which has numerous sinkholes and caves.
Most of the caves in the city have been sealed, but many springs are visible along the riverfront. Coal, brick clay, and millerite ore were once mined in the city. The predominant surface rock, known as St. Louis limestone, is used as dimension stone and rubble for construction. According to the United States Census Bureau, St. Louis has a total area of 66 square miles , of which 62 square miles is land and 4.1 square miles (6.2%) is water.