Spruce Run Reservoir Dam was a 1964 project of the New Jersey Water Supply Authority with municipal water supply as its primary use. The earthen dam is high, with a length of at its crest. The reservoir has a maximum capacity of and a normal capacity of . The reservoir is on land formerly used for farming and metal works over the past few centuries. Prior to European colonization of the area, the land underneath the reservoir was important to the local
Native Americans. Of the known Native American sites in Union Township, the majority were found on land inundated by the reservoir. The reservoir was formed by creating a series of dams that allowed two major sources, the
Spruce Run and its tributary, the Mulhockaway Creek, to flood the basin that was created. Water levels at the reservoir fluctuate greatly throughout the year, which has led to decreased vegetation in the
photic zone. This has a variety of repercussions for fish in the lake including decreased structure and habitat for juvenile fish. In recent years the reservoir's water levels have been lowered significantly for large portions of the year. ==State park==