Dorothy P. Miller played a central role in preserving the White Clay Creek Valley, a stream and floodplain habitat surrounded by rolling, forested hills that is home to more than 250 plant species, 30 species of mammals, and 185 species of birds. Rich in cultural history as well as natural resources, the valley has been inhabited by Native American Indians for at least 12,000 years, and Mason and Dixon spent many weeks surveying its lands. In the late 1960s, plans to dam the stream and create a reservoir threatened to flood one of the last areas of natural open space in New Castle County, but community and environmental groups organized against the project, which was finally removed from consideration in 1986. That successful conservation effort helped lead to the DuPont Company's donation of more than 1,700 acres to Pennsylvania and Delaware, forming lands that now comprise the White Clay Creek Preserve within White Clay Creek State Park and the bi-state preserve. Through lobbying legislators, organizing community support, and taking part in every major effort to protect the valley, Dorothy Miller helped expand protected land from just 24 acres to almost 3,000 acres and continued to advocate conserving as much of the stream valley as possible for future generations.