Exposed seams in the valley walls provided easy access to the coal. The earliest mines were built into the sides of the walls. This type of operation is called drift mining. As the demand for coal increased, shaft mines were constructed to harvest the coal deeper in the ground. Large mining complexes covered many square miles of the watershed.
Water infiltration happens with any type of mining.
Water within the rock layers seeps into holes created by mining. During drift mining, water was drained from the site either through the entrance hole or through passageways constructed in the walls of the valley. In larger mines, pumps were inadequate and bore holes were drilled to drain the tunnels. After World War II, new technologies reduced the demand for coal and many mines were abandoned. With the pumps shut down, mines filed with water and began draining into nearby waterways.