The Chicago Portage was an ancient portage that connected the Great Lakes waterway system with the Mississippi River system. Connecting these two great water trails meant 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. In the summer of 1673 members of the Kaskaskia, a tribe of the Illinois Confederation, led French explorers Louis Jolliet and Father Jacques Marquette, the first known Europeans to explore this part of North America, to the portage. A strategic location, it became a key to European activity in the Midwest, ultimately leading to the foundation of Chicago. In 1848, the water divide was breached by the Illinois and Michigan (I&M) Canal cutting through the portage. Created by a glacier The history of the Chicago Portage begins at the end of the last Ice Age. It was formed as the Wisconsin glaciation retreated northward about 10,000 years ago, leaving behind Lake Chicago (now called 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 encircles the lake's southern half, creating the Chicago Outlet River. This was a substantial river, comparable to today's Niagara River, and over time it carved the channel later used by the main and south branch 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 emerging lakes to drain even faster, and the Chicago Outlet River dried up leaving the gap in the moraine that created the Chicago Portage. This process created the Chicago Portage, a link between what became known as the Chicago River and what became known as the Des Plaines River. The point at which the portage crossed the low continental divide that separated waters flowing east toward Lake Michigan from waters flowing west toward the Mississippi River was a wetland that occupied the ancient 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.