The Riverside Hangars are semi-cylindrical-shaped hangar

Hangar

A hangar is a closed structure to hold aircraft or spacecraft in protective storage. Most hangars are built of metal, but other materials such as wood and concrete are also sometimes used...

s for aircraft installed at Saint Paul Downtown Airport

St. Paul Downtown Airport

St. Paul Downtown Airport , also known as Holman Field, is an airport just across the Mississippi River from downtown Saint Paul, Minnesota. It is one of several reliever airports in the Twin Cities operated by the Metropolitan Airports Commission...

 in 1942, immediately adjacent to the Mississippi River

Mississippi River

The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...

. Their important design features are their low-cost and that they were erectable quickly by unskilled workers. In Saint Paul

Saint Paul, Minnesota

Saint Paul is the capital and second-most populous city of the U.S. state of Minnesota. The city lies mostly on the east bank of the Mississippi River in the area surrounding its point of confluence with the Minnesota River, and adjoins Minneapolis, the state's largest city...

, Minnesota

Minnesota

Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...

, during World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, Northwest Airlines

Northwest Airlines

Northwest Airlines, Inc. was a major United States airline founded in 1926 and absorbed into Delta Air Lines by a merger approved on October 29, 2008, making Delta the largest airline in the world...

 employed up to 5000 people at the site, modifying new B-24 Liberator

B-24 Liberator

The Consolidated B-24 Liberator was an American heavy bomber, designed by Consolidated Aircraft of San Diego, California. It was known within the company as the Model 32, and a small number of early models were sold under the name LB-30, for Land Bomber...

 bombers, some of which received the highly classified H2X

H2X radar

H2X radar was an American development of the British H2S radar, the first ground mapping radar to be used in combat. It was used by the USAAF during World War II as a navigation system for daylight overcast and nighttime operations...

 radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

, which proved to be an invaluable tool in the European theater. Due to frequent flooding, proposals have been introduced to build a flood wall on the riverbank, but opposition and cost have thus far squelched the plans.