Nine Mile Run and the steam valley were deeply affected by the steel industry, most dramatically by the dumping of millions of tons of slag along the edge of the stream. In the begining of the 20th century, steel was king in Pittsburgh. This industry kept the people fed, but did a lot of damage to the environment. And although our watershed contained no mill, it was profoundly affected by the steel industry. Many working class watershed citizens worked at Carrie Furnace. Managers lived in the well-to-do houses in the watershed. Other industries sprung up in the region. Here in the watershed, Union Switch and Signal opened a bustling factory located in what is now the Edgewood Towne Centre, providing jobs for residents of the eastern boroughs.
Slag is a byproduct of steel. When the furnaces "bake" ore to obtain steel, slag is what is left over. It is made up mostly of aluminum and silica. For every ton of steel produced, at least a quarter ton of slag is left over. It needs to go somewhere.
In 1922, the Duquesne Slag Company bought 94 acres of the Nine Mile Run stream valley. Over the next decades, they bought more and more land along the stream. Despite the area being zoned residential, Duquesne Slag continued dumping slag into the valley for the next half century. It is estimated that 200 million tons of slag now rest on the banks of Nine Mile Run.
But despite slag's high alkalinity, porous nature, and general unfitness for growing things, nature is persevering and plants are slowly revegetating the slag heap on their own. Humans are also helping to improve the slag heap by building new housing, adding vegetation, and creating a 100-acre greenway connecting Frick Park to the Monongahela River.
Slag Heap, photo by John Moyer.