A waterfall flows in downtown Portland, Ore., ribbons and rivulets of water cascading over slabs of rough, reddish concrete into pools filled with wading children in the summer. Down a tree-lined path, great planted hills pop from the sidewalk. A stepped basin opens up between buildings, looking like a natural spring bursting through the pavement.
These bold environments, strung across an eight-block section in the city center, were designed by the modernist landscape architect Lawrence Halprin and his firm between 1965 and 1970, and listed on the National Register of Historic Places in 2013. They are celebrated, along with more than two dozen other parks, pools and gardens, in “The Landscape Architecture of Lawrence Halprin,” an exhibition in Washington commemorating Mr. Halprin’s centennial that runs through April 16.