as St. Louis limestone, is used as dimension stone and rubble for construction. 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. The city's eastern boundary is the Mississippi River, which separates Missouri from Illinois. The Missouri River forms the northern line of St. Louis County, except for a few areas where the river has changed its course. The Meramec River forms most of its southern line. Climate The urban area of St. Louis has a humid subtropical climate (Köppen: Cfa); however, its metropolitan region even to the south may present a hot-summer humid continental climate (Dfa), which shows the effect of the urban heat island in the city. The city experiences hot, humid summers and chilly to cold winters. It is subject to both cold Arctic air and hot, humid tropical air from the Gulf of Mexico. The average annual temperature recorded at nearby Lambert–St. Louis International Airport, is . temperatures can be seen on an average 3 and 1 days per year, respectively. Precipitation averages , but has ranged from in 1953 to in 2015. The highest recorded temperature in St. Louis was on July 14, 1954, and the lowest was on January 5, 1884. 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 (often rapid) melting of thick snow cover upstream on the Missouri or Mississippi Rivers can contribute to springtime flooding. Flora and fauna Before the founding of the city, the area was mostly prairie and open forest. Native Americans maintained this environment, good for hunting, by burning underbrush. Trees are mainly oak, maple, and hickory, similar to the forests of the nearby Ozarks; common understory