Flood Plains

Human activity has extensively modified the floodplains within the park over time, through farming; dredging; and construction of dams, wells, impounding basins, canals, rail lines, roads, and power lines. However, based on Federal Emergency Management Agency (FEMA) mapping, the Schuylkill River and Valley Creek still retain both 100- and 500-year floodplains to some degree.

Schuylkill River

Extensive floodplains are located along the Schuylkill River through the park. On the south bank, the 100-year floodplain generally lies between the railroad and the river. On the north side, the floodplain slopes gently up to an escarpment forming the edge of the upper river terrace. In addition, smaller floodplain areas are found on the north side along several tributaries of the Schuylkill, including Lamb Run. Elevations of the 100- year floodplain for the Schuylkill River range from 81-92 feet, based on the National Geodetic Vertical Datum of 1929 (NGVD). Poorly drained, deep, alluvial soils comprise the surface layer within the floodplain areas (FEMA 1996a, FEMA 1996b).

In the 1940s, a series of impounding basins were built along the Schuylkill as part of the Schuylkill River Reclamation Project. The basins were installed to remove coal silt from the river. Two of these basins were located below Walnut Hill in what is now the park. By the mid 1980s, these impoundments were no longer needed, but the massive stone and earthen walls remain, lessening the area’s ability to function as a floodplain. Additionally, deposits of coal silt remain on the surface of most of the floodplain, ranging in depth from one to over five feet.

Valley Creek

Floodplains are also found along Valley Creek, ranging in elevation from 96 to 120 feet NGVD with the largest floodplain located in the southwestern portion of the park (FEMA 1996a, FEMA 1996b). However, the floodplains are generally not as expansive as those along the Schuylkill River due to the steep slopes surrounding Valley Creek through much of the park. In fact, the lack of a floodplain in the most narrow parts of the stream corridor (near its confluence with the Schuylkill River) is one reason why Valley Creek is undermining PA Route 252 (Valley Forge NHP 2001b).

Within the park, in the area between the covered bridge and the confluence with the Schuylkill, most of the creek’s former floodplain is filled with the sediment that accumulated behind historic dams and one existing 20th century dam. The sediments greatly reduce the capacity of the floodplain to accommodate stormwater, and also contribute to ongoing sedimentation.

For approximately one-third of its length through the park, Valley Creek is bounded on both sides by steep hills – Mount Misery and Mount Joy. Given the high volume of stormwater flowing into the park and this steep topography, the creek within the park is particularly susceptible to the effects of increased runoff, including flooding, streambank erosion, and pollution.