Illinois State Museum

globe and map showing location of swamp

Southern Swamp - Climate and Geology


The climate of the Cache River Basin in 1673 was similar to the climate of southern Illinois today. Scientists estimate an average annual high temperature of 19º Celsius (about 67º F), average low temperature of 88º C (about 46º F). There were 190–200 frost-free days annually, and the average rainfall was 121 cm (48 in).

The Cache River Basin has the warmest climate in Illinois. The northern range of many southern species extend into this area.


The Cache River drains almost all of southern Illinois’ watersheds . The upper Cache River begins in the Shawnee Hills. At its headwaters, it is a steep, fast-flowing river that has cut deep into the sandstone bedrock. the lower Cache River meanders through a flat landscape of swamps, sloughs, and oxbow lakes. The 1806-1809 U.S. Land Survey described this area as “a drowned land.”