100 years ago, upper class Boston-area residents spent summers in Rivermoor on Third Cliff in Scituate. They included dentists, lawyers, executives, suffragists, and the first American to make prefabricated houses. They and their families came for the area’s ocean views, beaches, and cooling breezes on the wraparound porches of Colonial Revival houses. George Welch built these houses and turned what had been farmland since Native American times into a select summer colony.