The Chicago Portage was an ancient portage that connected the Great Lakes waterway system with the Mississippi River system. This connection provided comparatively easy access from the mouth of the St. Lawrence River on the Atlantic Ocean to the Rocky Mountains and the Gulf of Mexico. The approximately six-mile link had been used by Native Americans for thousands of years during the Pre-Columbian era for travel and trade.

During the summer of 1673 members of the Kaskaskia, a tribe of the Illinois Confederation, guided French explorers Louis Jolliet and Jacques Marquette—the first known Europeans to explore this part of North America—to the portage. A strategic location, it became important to European exploration in central North America, resulting ultimately in the founding of Chicago.

The portage crossed waterways and wetlands between the Chicago River and the Des Plaines River, through a gap in the Valparaiso Moraine. In 1848, the water divide was breached by the construction of the Illinois and Michigan (I&M) Canal through the portage; this was deepened and widened in 1900 by the Chicago Sanitary and Ship Canal which was also used to reverse the water's directional flow.

The history of the Chicago Portage begins at the end of the Last Ice Age. It was formed as the Wisconsin glaciation retreated northward, beginning about 14,000 years ago, leaving behind Lake Chicago (the ancestor of Lake Michigan), which was created from the glacier's meltwater.

As the glacier melted and retreated, the water in Lake Chicago rose until it overflowed the southwestern edge of the Valparaiso Moraine, which surrounds the lake's southern half, creating the Chicago Outlet River. This was a substantial river, comparable to the present Niagara River, and over time it carved the channel used by the main and south branches of the Chicago River, the Des Plaines River, and the terrain that became the Chicago Portage.

As the glacier continued to retreat, it opened another outlet far to the east that became the St. Lawrence River. This allowed the lakes to drain faster in that direction, and the Chicago Outlet River dried up, leaving a gap in the moraine that served the Chicago Portage.

The Chicago Portage linked the Chicago River's South Branch and the Des Plaines River. The point at which the portage crossed the low water divide that separated waters flowing east toward Lake Michigan from waters flowing west toward the Mississippi River was a wetland that occupied the former stream bed of the Chicago Outlet River. Early settlers called this marshy area "Mud Lake". The total length of the portage was about six miles.

Mud Lake could be wet, dry, marshy, or frozen, depending on the season and the weather, making it at times a difficult albeit very valuable transportation route. During very wet weather the water level in both the Des Plaines River and the Chicago River would rise to the point that Mud Lake was flooded, and travelers could traverse the entire six miles by canoe. During dry weather, particularly in late summer, the water level would be much lower, and it was necessary to pull out canoes at some point and carry them and all supplies through the area of Mud Lake.