PRE-SETTLEMENT BISON SKELETAL REMAINS IN THE DES MOINES RIVER, SOUTHWESTERN MINNESOTA

Bison were locally extirpated in southern Minnesota by the time permanent European settlements were established. The discovery of bison bones on sand bars of the Des Moines River in Jackson County, MN offers an opportunity to examine river processes over relatively short timescales, to explore taphonomic alteration of these remains, and to probe the interaction between humans and bison just prior to the modern era.

The Des Moines River traverses unconsolidated glacial and post-glacial sediment, producing a river valley with steep banks and broad terraces. Sandbars consist of sand- to cobble-sized particles, which include rocks, bones, artifacts, and older fossils, including petrified wood. Most of the geologic material in the sandbars were carried to the area by glaciers and pre-dates the development of a modern landscape. Some of the bone material in the river comes from animals common in the modern landscape (e.g., beaver, deer, and horse). However, bison bones are found in the river, in a state of active transport with sand and gravel bars and are unexpectedly abundant in these settings. Episodes of transport occur when sediment is mobilized, during river flood. Bones are easiest to find during the summer or autumn, when low river levels permit access to sandbars by kayak or on foot.

Bison bones were discovered near a canoe ramp in Petersburg, MN in 2012. This discovery led to several exploratory trips by kayak and three intensive collecting intervals. We have now collected and catalogued 865 bones, the majority of which are bison. These bones, including tarsals, limbs, shoulders, ribs, vertebrae, and skulls, are discolored, abraded from transport, and partially mineralized. In addition, some bones show evidence of human modification. Bison bones are most abundant in the stretch of river between Jackson and Petersburg but have been found as much as 30 km upstream.

Radiocarbon ages indicate that the bison bones span a range of ages between 200 and 1000 years BP. It is likely that the bison died in or near the river and may have been killed or butchered by humans. Episodic burial and exhumation of sandbars then transported the remains downstream. Bones were likely concentrated in pools and behind natural and manmade obstructions in the river. As these obstructions were removed or eroded, bones again entered the active river.