In 1927 the Mississippi River flooded like crazy and killed hundreds of people while doing insane levels of property damage. The following year, Congress passed a bill to let the Army Corps of Engineers study and alter country's rivers to prevent or reduce such flooding in the future. The Corps did some studying and small scale model building, bit then in 1943 they started on a huge project: building a 1/2000 scale model of the Mississippi River and all water sources that feed into it. That meant making an accurate scale model of the waterways of about half the continental United States. They used German prisoners of war from WWII as labor to build the thing, and it took about 6 years. When it was done it covered 200 acres of land, and one could only see the whole thing at once by climbing to the top of a 4 story tall observation tower. Engineers used it to test flooding conditions, and it was accurate enough to prevent major damage from at least one major flood that would have caused millions of dollars in damage. The model was not officially closed down until 1993.
Oh, and as a follow-up to the above fun historical fact, even today computer models are not as good as physical models for showing how fluids will move and react, especially in natural conditions. That giant scale model was far more accurate than a lot of the computer programs in use today, and physical models are still created by the Army Corps of Engineers and other organizations to simulate things that our computers cannot do.
