More than 30 miles of shoreline along the James and York rivers help form the boundary of the park. In addition, approximately 24 miles of perennial streams and 30 miles of intermittent streams and drainage's flow through the park. Numerous freshwater and brackish creeks also flow through the park. The Colonial Parkway passes among upland and tidal streams as well as freshwater and brackish ponds. A freshwater spring and a small creek are at Green Spring plantation. Also, a series of springs and seeps originate on Yorktown Battlefield. Most of the water bodies and wetlands in the park have major portions of their drainage basin upstream and outside of park boundaries. Therefore, activities outside of the park can have a detrimental effect on water quality within the park (oil spills, erosion and sedimentation, chemical use).