The Chicago River South Guidewall light is located at the mouth of the Chicago river at the end of the south pier and east of Chicago Harbor Lock.

Prior to being placed on the south guidewall, the metal tower had served as Kewaunee Pierhead Lighthouse in Wisconsin from 1912 to 1931.

Upon the construction of the Harbor Locks, the Chicago Harbor southeast guide wall was built, and the present Guidewall Light tower was established at its eastern end of the south pier in 1938 to help guide vessels into the lock.

In 1871, due to stagnant conditions in the Chicago river causing multiple epidemics, the city of Chicago constructed a canal from the Chicago river to the Des Plains River, reversing the flow of the Chicago River, causing the flow, which was previously to Lake Michigan, to begin flowing into the Des Plains river which would then flow to the Mississippi river and subsequently to the Gulf of Mexico.

This increase of water flow from this diversion of the lake water into these river systems caused downstream flooding. The neighboring states complained about increased flooding along the banks of the rivers that were affected by the division of the Lake Michigan water. A new canal was completed and put into use in January 1900, which canal system was engineered better, but, it was still prone to cause downstream flooding whenever there was heavy rains and or higher lake levels.

These affected states then jointly filed a complaint in a US Supreme Court which in 1930, resulting in a court order to decrease and limit the amount of water that was being diverted from Lake Michigan into the Chicago River. To comply with the court order, the Chicago Sanitary District with the Army Corps of Engineers, began in 1934 to build a set of locks at the entrance to Chicago river.

The Chicago Harbor Lock is a pound lock located in Chicago, Illinois, at the mouth of the Chicago River separating Lake Michigan from the Chicago River.

Operated by the U.S. Army Corps of Engineers, the lock was designed, built and completed in 1938 by the Sanitary District of Chicago as a component of the project to limit the flow of the Chicago River and is one of two entrances from the Great Lakes to the Chicago Area Waterway System, the other entrance being the T.J. O′Brien Lock and Dam on the Calumet River.

The Chicago Harbor lock chamber is 600 feet long, 80 feet wide, and 22 feet deep and can accommodate up to 100 vessels at once. The lock requires 12 to 15 minutes to cycle through a typical water level difference of two to five feet. Water level is controlled via gravity through partially opened lock gates.

The Chicago Harbor Lock opens around 10,000 times each year, making it the fourth busiest lock in the nation, and the second busiest lock in the country. Over 50,000 vessels, carrying around 900,000 passengers, pass through the lock annually.

Isophase means equal periods of on and off.

Site and tower closed.