[5] The Second Treaty of Prairie du Chien, signed by General John McNeil Jr., Colonel Pierre Menard, and Caleb Atwater for the United States on June 29, 1829, references four sections of land being reserved for Chief Awn-kote and his band of 171 villagers "at the village of Saw-meh-naug along the Fox and Illinois Rivers".

[6][7] The Saw-meh-naug, along with the other Pottawatomie of the Illinois River (known as the Prairie Group), as well as the Sauk and the Chippewa, had aided the U.S. in the Black Hawk War.

After the war, as a thanks to the Pottawatomie, the U.S. Government signed a treaty allowing the Prairie Group tribes to continue to hunt on U.S. land, outside of their allocated reservations.

However, due to "rumors of the Pottawatomie being restless and committing depredations in the northern part of the state spreading to central Illinois", and white residents of the state feeling uncomfortable, the Governor "gave the Prairie Group an ultimatum to leave Illinois".

[7] A few months later, in the spring of 1834, the first white settler in DeKalb County had set up shop along a tributary to the Fox River, and the name Somonauk was moved from the area of present-day Ottawa, to where Chicago Road, Governor Beveridge Highway, and Somonauk Creek meet.

The simple log cabin was used as a station house along the Chicago-Galena mail route between Aurora and Genoa, and would eventually become property of the Beveridge family.

This cabin was the first post office in Somonauk Township, and the area both east and west of Somonauk Creek would eventually have 30 settlers, establishing a Presbyterian Church in the same area as the present-day church, and a school house directly across from it on the west side of Governor Beveridge Highway.

In 1851, the CB&Q railroad finally made its way through the township, and in 1853, built a station along the line 5 miles south of the early settlement (now known as "Franks"), just north of the LaSalle/DeKalb County border.

[12] Sannauk Forest Preserve serves as the unofficial boundary between Somonauk and Sandwich, which resides to the east.

The forest preserve, owned and managed by DeKalb County, sits along Somonauk Creek, a tributary of the Fox River.

When the housing market crashed, the city had already incorporated a large portion of DeKalb county north of town ranging from the Sandwich Township border to Governor Beverage Highway, remaining south of Pratt Road.