Chicago Portage

Chicago Portage
Maps of the Chicago Portage, on a sign at Chicago Portage National Historic Site
Elevation	589 ft (180 m)
Traversed by	Mud Lake (historic), Illinois and Michigan Canal (historic), Chicago Sanitary and Ship Canal, several railroads, numerous roads including I-55 (Stevenson Expressway)
Location	(historic) 3100 West 31st Street, Chicago, Cook County, Illinois, US
Range	Valparaiso Moraine
Coordinates	41°50′14″N 87°42′8″W / 41.83722°N 87.70222°W / 41.83722; -87.70222

The Chicago Portage is a water gap connecting the watersheds (BrE: drainage basins) and the navigable waterways of the Mississippi River and the Great Lakes. It cuts through the Valparaiso Moraine, crossing the Saint Lawrence River Divide that separates the Great Lakes and Gulf of St. Lawrence watersheds from the Gulf of Mexico watershed, making it one of the most strategic points in the interior of the North American continent. The saddle point of the gap is within the City of Chicago, and the Chicago Portage is a reason Chicago exists and has developed to become the immensely important city that it is, ranking 7th in the world in the 2014 Global Cities Index. The official flag of the City of Chicago includes four red stars symbolizing city history, separating two blue stripes symbolizing the waters that meet at the city.

A principal feature of the Chicago Portage water gap is that water can flow through it in either direction across the continental divide. It has flowed from east to west, west to east, or not at all. There have been two long-term reversals, and short-term reversals still happen today. Initially, water flowed from east to west starting about 10,000 years ago. About 3,000 years ago, this flow mostly stopped, and the Chicago Portage became a wind gap, except during floods when it flowed from west to east. In the year 1900, it was reversed to again flow continuously from east to west, except during floods when it can be reversed to flow from west to east.

The Chicago Portage was formed as the Wisconsin glaciation retreated northward about 10,000 years ago, leaving behind Lake Chicago consisting of its meltwater. Lake Chicago found its only outlet at its southwestern edge, where it overflowed the Valparaiso Moraine which encircles the southern half of the Lake Chicago/Lake Michigan basin, creating the Chicago Outlet River. This was a substantial river, carving the channel later used by the main and south branches of the Chicago River, and the Des Plaines River. Eventually the glaciers melted far enough northward that Lake Chicago was connected to Lake Huron, a combination sometimes called Lake Michigan-Huron, resulting in a significant drop in the water level. Until perhaps 3,000 years ago, the combined lakes had two outlets, one at Chicago, and the other via the St. Clair River at Port Huron, Michigan. Eventually the St. Clair River outlet eroded slightly more quickly, resulting in stream capture of the Chicago Outlet River as the water level of Lake Michigan-Huron gradually declined. However, until man-made canals and levees were built within the past 170 years, during periods of heavy rain or downstream ice dams, the Des Plaines River could still flood eastward through a marshy area of the bed of the former Chicago Outlet River known as Mud Lake, across the continental divide into the Chicago River and then into Lake Michigan.