The Grand Calumet River is a 13.0-mile-long (20.9 km) river that flows primarily into Lake Michigan. Originating in Miller Beach in Gary, it flows through the cities of Gary, East Chicago and Hammond, as well as Calumet City and Burnham on the Illinois side. The majority of the river's flow drains into Lake Michigan via the Indiana Harbor and Ship Canal, sending about 1,500 cubic feet (42 m³) per second of water into the lake. A smaller part of the flow, at the river's western end, enters the Calumet River, and through the Illinois River ultimately drains into the Mississippi River.

Today, a large portion of the river's flow originates as municipal and industrial effluent, cooling and process water and storm water overflows. Although discharges have been reduced, a number of contaminants continue to impair the area. The river is among the country's most severely polluted, and as of 2015 was in the late stages of a major dredging project to remove thousands of tons of contaminated sediment and rehabilitate the river ecosystem.

The Grand Calumet is divided into an East and West branch, on the respective sides of the Indiana Harbor Canal. The East Branch, which drains entirely into Lake Michigan, rises in Marquette Park in Gary's Miller Beach neighborhood, passing through a series of three lagoons - two of which are partially located within the Miller Woods unit of Indiana Dunes National Park - before flowing through the industrial zone of Gary. It leaves Gary by passing under Cline Avenue, and then defines the boundary between East Chicago and Hammond until reaching the Indiana Harbor Canal near Kennedy Avenue.

The West Branch drains partly into the Indiana Harbor Canal and partly into the Calumet River, which drains into the Chicago Area Waterway System and ultimately into the Illinois River. The divide between the part of the West Branch flowing into the Illinois River and the part flowing to Lake Michigan is in Hammond, Indiana, near the Illinois-Indiana state line. The West Branch joins the Little Calumet at Burnham, Illinois.

In addition, the Indiana Harbor Canal itself is sometimes treated as a part of the Grand Calumet, which brings the river's total length to 16.0 miles (25.7 km).

The Grand Calumet flows through the Calumet Lacustrine Plain, a region of low, undulating sandy terrain adjoining Lake Michigan. Groundwater contributions to the river, which account for less than 10% of total flow, come from the Calumet Aquifer, an unconfined water-table aquifer. The Calumet Aquifer is less than 10 feet below the surface in most areas, and has an average thickness of 40 feet.

Prior to the industrialization of the Chicago area and the creation of the Chicago Area Waterway System, the Grand Calumet flowed into Lake Michigan at Marquette Park in Miller Beach, near where its headwaters now lie. In this original configuration, the Grand Calumet included much of what is now the Little Calumet River, while the original "Little Calumet" (today the Calumet River) was a minor stream which drained Lake Calumet and Wolf Lake into Lake Michigan. This original river is sometimes called by the older name Konomick or Kennomick to distinguishing it from the Grand Calumet of today; the term comes from the Miami kinwikami, or "long water river".

This arrangement began to shift in the early 19th century, either in 1805 or between 1809 and 1820, when some combination of flooding and human excavation opened up a channel allowing the upper Grand Calumet (now Little Calumet) to drain directly into Lake Calumet. This created the basic definitions of the rivers as they exist today, as what had been the upper reaches of the Grand Calumet now became part of the Little Calumet. The Grand Calumet at this point still flowed from west to east, emptying into the lake at Miller Beach. But by 1872, as more and more of the river's flow was captured by the Calumet River, its original mouth at Miller Beach had been completely blocked by sandbars, totally reversing the flow of the river.