High Quality Content by WIKIPEDIA articles! The Sac River is a river in southwest Missouri. It is 107 miles (172 km) long, with headwaters in Lawrence and Greene counties; the headweaters join near Greenfield, then flow north through the Ozarks, to the Osage River, ending just above Osceola, Missouri in Truman Reservoir. Large portions of the Sac River and the Little Sac River are inundated by Stockton Lake. The Big Eddy Site, an archaeological dig, is along the Sac River within Cedar County. Eleven feet of river sediment at the site provides a stratigraphy that suggests more than 10,000 years of nearly constant occupation by American Indians, potentially pre-dating the Clovis culture and contributing to the knowledge of the Dalton and San Patrice cultures.