==Geography==
Jefferson County is located in eastern Missouri, within the northeastern foothills of the Ozark Mountains. Much of its boundaries is determined by rivers, with the Mississippi making up its entire eastern border, the Meramec (which flows into the Mississippi just east of Arnold) most of its northern boundary, and the Meramec's tributary, the Big River, defining the southwestern corner. It is bordered on the east by the state of [[Illinois]], on the southeast by [[Ste. Genevieve County, Missouri|Ste. Genevieve County]], on the south by [[St. Francois County, Missouri|St. Francois County]], on the southwest by [[Washington County, Missouri|Washington County]], on the west by [[Franklin County, Missouri|Franklin County]], and on the north by [[St. Louis County, Missouri|St. Louis County]].
The county has a total area of 664.29 square miles, including 656.63 of land and 7.66 of water.<ref>https://web.archive.org/web/20131021170230/http://www.census.gov/geo/maps-data/data/docs/gazetteer/counties_list_29.txt</ref> It is dominated by hilly terrain, some of it comparatively rugged, especially in the east near the Mississippi and in the southwest as an extension of the higher St. Francois Mountains, while other land in the interior is more gently rolling in nature. The highest peak is Vinegar Hill near the St. Francois County line, with a height of 1,060 feet above sea level; the lowest elevations come in the narrow floodplains along the major rivers. Along the bank of the Mississippi in the southeastern corner, it is only about 390 feet.<ref>https://www.peakbagger.com/peak.aspx?pid=6570</ref> In recent decades, there has been considerable urban and suburban development in the northern half of the county, particularly around the Arnold area, while the southern half is still much more rural in character.