Building a model like this is a complex project requiring precise scaling and ultra-precise shaping of the terrain. I found some photographs that help explain how a scale physical hydraulic model like this is constructed.
At the Mississippi Basin Model, the original earth work, excavating canals, and installing piping was performed by German prisoners of war. The Germans were repatriated in 1946, and civilian employees completed the rest of the model. The template method of shaping the terrain was used between 1946 and approximately 1953. But the expansive soil in the area required a shift to the contour method of construction (not shown in these photographs).
Historical black and white photographs courtesy of the Coastal and Hydraulics Laboratory, Engineer Research and Development Center, US Army Corps of Engineers.