Seneca Dam

Beginning in the 1880s a series of proposals were made to use the Potomac River's hydropower potential, which arises from the steep drop of the river at Great Falls as it crosses the Atlantic Seaboard fall line, dropping to sea level over about 10 miles (16 km). Frequent flooding and a desire to secure a reliable water supply for Washington, D.C. increased the attractiveness of a major dam on the Potomac.

A detailed plan for a major dam on the Potomac at Great Falls was first advanced by the Corps of Engineers in a 1921 report, which proposed a concrete gravity dam which would include a hydroelectric powerplant. Reservoir capacity was projected at 300,000 acre-feet (0.37 km³) for a 24,000-acre (9,700 ha) reservoir with a reservoir at 215 feet above sea level.[1] Power generation capacity was planned at 105 MW[2] and total cost was projected at $18,616,000 in 1921.[3]

Dams proposed in the Potomac River basin in 1945

The Riverbend Dam was to be a 119-foot (36 m) dam at the westward bend of the Potomac just above Great Falls. Its reservoir was to extend nearly to Harpers Ferry, with branches extending up the Monocacy River past Frederick and major branches on streams in Loudoun County, Virginia. The dam was to incorporate small locks to allow pleasure boats to reach Harpers Ferry. After intense public opposition the Riverbend project was shelved in favor of less ambitious proposals[4]

The final proposal for a lower Potomac dam was Seneca Dam. The concept developed in the 1950s and was proposed by the Corps of Engineers in its final form in the 1963 Potomac River Basin Report. Answering objections to the visual intrusion of the Riverbend Dam at Great Falls, Seneca Dam was placed at Blockhouse Point, about 1.6 miles (2.6 km) downstream from the mouth of Seneca Creek, about 4.5 miles (7.2 km) upstream from the Riverbend site and just downstream from the Chesapeake and Ohio Canal's Dam Number 2. No immediate hydroelectric development was planned, the dam was meant to provide a water source, to control floods, and to provide recreation. The Corps of Engineers assumed that water quality would continue to decline under the effects of upstream sewage discharge, especially during periods of low river flow, and opined that unless sewage was diverted by pipeline to the Chesapeake Bay, a dam would be necessary to dilute the effluent to an acceptable level.[8] However, the Corps projected that the Seneca reservoir would not be able to assure sufficient water flow and quality without support from upstream dams on tributaries.[9]

The dam was planned as a straight-crested concrete gravity dam with a central overflow spillway regulated by 27 gates. Provision for future power generation was to be included in the south abutment, with water supply provisions on either end of the dam. No lock was to be provided for small craft.[10] For flood control the project was designed to pass 150,000 cu ft/s (4,200 m³/s) of a 400,000 cu ft/s (11,000 m³/s) flood to minimize damage downstream. Drawdown from May to September was expected to be about 5 feet (1.5 m), with another 7 feet (2.1 m) for the rest of the year. The pool would have varied from 38,700 acres (15,700 ha) at maximum flood pool to 24,000 acres (9,700 ha) at full conservation pool, and 9,100 acres (3,700 ha) at minimum pool. Construction was expected to take six years. The reservoir was planned to store 550,000 acre-feet (0.68 km³), of which 50,000 acre-feet (0.062 km³) were allocated for sedimentation. An additional 643,000 acre-feet (0.793 km³) were available for flood storage.[8]

The reservoir was projected to cost more than $100 million in 1962 dollars. The project was opposed by Maryland and Virginia, as well as by conservation organizations and fell out of favor with the Corps of Engineers in 1969.[6] As with the Riverbend proposal, the reservoir was to spread along tributary creeks and reservoirs, in some cases considerable distances. On the Virginia side, Sugarland Run would have been flooded to Herndon, Broad Run to Sterling, Goose Creek to Evergreen Mill and Catoctin Creek to Taylorstown. On the Maryland side, Seneca Creek would have been flooded past Dawsonville, and the Monocacy River would have been flooded past Frederick to Ceresville. The main river pool would have extended to Sandy Hook, Maryland. Maps of the reservoir show that portions of Point of Rocks and Buckeystown, Maryland would have been flooded by the full flood pool, and most or all of Lilypons and Seneca in Maryland, Limestone School and Fairview in Virginia and White's Ferry in Maryland and Virginia. The lower portion of the Ball's Bluff battlefield would have been submerged, as well as the Monocacy Aqueduct.[11]

Roads and railroads were expected to be flooded by the reservoir and would have had to be replaced. The 1963 report described the relocations as "extensive." Road relocations were budgeted at $8,500,000 and railroads at $10,040,000. About 21,000 acres (8,500 ha) of wildlife habitat was expected to be inundated, including the McKee-Beshers and Dierssen wildlife management areas. 13,120 acres (5,310 ha) of replacement habitat was expected to be required to mitigate habitat loss. The project was judged to have little effect on agriculture and forestry. 460 families were expected to be displaced.[8]

• Tocks Island Dam controversy