Why did the Plateau Indians construct pit houses?

They had well established winter villages built on the banks of a major waterway.  Homes were built partially underground. Some were little round houses with dirt roofs.  Some were long lodges. They were both pit-houses.

To build a pit-house, first you dug a pit about six feet deep.  Some pit-houses were built for a small family group and perhaps were only 20 feet in diameter. But some were much larger - as much as 60 feet wide and 100 feet long.  The small ones were usually oval in shape. The large ones were usually rectangular. But construction of all pit-houses started by digging a pit that was the outline of the house.

Once you had dug the pit, next you piled up rocks to make walls. Then you added some wooden posts to support a roof.  Roofs were made of everything from planks to woven reeds. (Baskets and mats were also made out of reeds.)  

Explanation:

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the main concern with the first draft of the articles of confederation was too much power in the state governments.