William Dunbar (November 18, 1749 – October 16, 1810) was a Scottish-born American merchant, plantation owner, naturalist, astronomer and explorer.

William's father thought him a sissy and worried about his health, because he was quiet and serious-minded, unlike his half-brothers, who hunted, fished, and drank.

He then went to Pensacola and obtained a grant of land near Baton Rouge, which was at the time called Fort New Richmond, from the British governor.

In 1807 he wrote Charleston merchants, Thomas Tunno and John Price, to purchase a shipload of African slaves excepting those from 'the Iboa nation' and those 'nearer the coast, such as Bornon, Houssa, Zanfara, Zegzeg, Kapina, Tombotoo, all or near the river Niger'.

In 1799, Daniel Clark introduced Dunbar to Thomas Jefferson (then Vice President) through a letter, saying "for Science, Probity & general information [he] is the first Character in this part of the World".

The trip was drastically altered due to friction with the Osage Indians and Spanish colonial officials, resulting in a shorter journey.

Although Dunbar's failing health prevented him from participating in the expedition, he nevertheless was charged by Jefferson with the task of organizing it, together with Secretary of War Henry Dearborn.

This expedition was led by astronomer/surveyor Thomas Freeman and medical student Peter Custis, and it reached 615 miles (990 km) up the river before being halted by a Spanish military force.

[3] "It provided Americans with the first scientific study of the varied landscapes as well as the animal and plant life of early southern Arkansas.

[3] The Hunter-Dunbar expedition was extremely important to the future for the United States because the information garnered was essential to the American expansion westward.

[3] The success of this expedition is based upon Hunter and Dunbar's extensive journal entries, their scientific observations and a few geological experiments that were conducted during the trip".

Dunbar and Hunter were not the first to travel the Ouachita River or to taste the waters of the hot springs, nor were they the first to describe the region in journals or publications.