Article and photos by John W. Jengo

The Robert Street Dam, located at Raritan RM 27.9, was near the eastern end of Duke Island Park in Bridgewater and Hillsborough Townships, Somerset County, New Jersey.  The original dam at this location was constructed circa 1915 (referred to historically as the Dead River Dam or the Bradley Gardens Dam), but no written documentation has been found regarding the purpose of this structure and the entity that funded its construction remains unknown.  My reconnaissance of the dam environs uncovered two concrete retaining walls on the south bank just upstream of the dam along the shoreline of Duke Farms. These features appear to have been the anchorage for a debris deflection boom, which may suggest that the so-called Dead River (a narrow U-shaped channel south of the Raritan River) may have been an irrigation canal that watered former agricultural fields on the Duke Farms property.  An engineering report from 1963 revealed that about 100 feet of the Dead River Dam had overturned, settled, or had seriously deteriorated by the early 1960s, most likely due to water infiltration and scouring of the underlying foundation.  

A substantial reconstruction of the dam was implemented in 1964 for the stated purpose of recreational boating, presumably to create an impoundment adjacent to the newly opened Duke Island Park upriver of this location.  During the reconstruction, the remnants of the original dam were encased between parallel, horizontal waler beam-supported steel sheet piles, spaced 17.5 feet apart, which were driven into bedrock.  Upon completion, the reconstructed dam was approximately 7.5 feet high, 255 feet long and had a width of 23 feet, which included a 17.5-foot-wide rock backfilled section between the sheet piles that entombed the original dam.  Unfortunately, the Robert Street Dam (along with the Headgates Dam located two miles upriver) became one of the most significant obstructions to migratory fish passage in the Raritan River watershed, and the dam became the site of drowning fatalities of recreational boaters that were swept over the dam, trapping them in the deadly hydraulic jump and reverse roller that occurred at the base of the dam spillway.

To remove the dam, numerous technical and permitting challenges had to be resolved over the 3-year planning and permitting period, including how to circumvent the inadequate Robert Street Bridge load capacity that precluded heavy equipment access to the dam site. To solve the access problem, careful daily monitoring of the river’s stage height over several years was conducted and the resultant data provided an indication of how low the river level needed to drop for a submerged sand shoal upstream of the dam to be less than 3 feet deep, enabling heavy equipment to be driven down the river from Duke Island Park to the dam. Technical obstacles that were overcome during the 5-week dam removal included managing the work around frequent upriver Round Valley Reservoir releases, the laborious extraction of over 300 individual sheet piles using two different hydraulic vibratory driver/extractors, and conveyance of over 700 tons of dam demolition concrete and 70 tons of steel sheet piles offsite for recycling.

The removal of the Robert Street Dam, in conjunction with the removal of the Calco Dam and the Nevius Street Dam downriver, eliminated the physical barriers to anadromous fish passage along nine miles of the main stem of the Raritan River between RM 20.9 and RM 29.9 and the lower 1.5 miles of the Millstone River tributary. These removal projects restored access to historically significant spawning grounds for American shad (Alosa sapidissima), hickory shad (Alosa mediocris), blueback herring (Alosa aestivalis), and alewife (Alosa pseudoharengus) in New Jersey’s largest interior watershed and river system. My post-dam removal observations verify the elimination of the filamentous mat algae and stagnant water conditions behind the former Robert Street Dam and have shown that the natural river process of transporting sediment, woody debris, and nutrients has been restored.   An outstanding example of this recovery would be the lifeless sand shoal that was used to access the dam from upstream, which was drowned for nearly 100 years in a de-oxygenated, algae-covered impoundment, that was transformed following the dam removal to a vibrant island habitat supporting plants, insects, and birdlife.

John W. Jengo, PG, LSRP is a licensed Professional Geologist in several Northeastern and Southeastern states and a Licensed Site Remediation Professional in New Jersey. John works as a Principal Hydrogeologist in an environmental consulting firm in southeastern Pennsylvania. He has degrees in geology from Rutgers University (1980) and the University of Delaware (1982). Over the last 30 years, he has conducted the characterization and remediation of large, complex contaminated industrial sites throughout New Jersey, Pennsylvania, and Delaware. He played a key role in Natural Resource Damage (NRD) assessments that led to groundbreaking legal settlements to remove numerous low head dams on the Raritan and Millstone Rivers to restore historically significant migratory fish spawning runs. As technical project manager, he planned, permitted, and successfully managed the removal of the Calco Dam, the Robert Street Dam, and the Nevius Street Dam between 2008-2013, and the removal of the Weston Mill Dam on the Millstone River in 2017, along with leading the archaeological investigation of the former Weston Mill in the Borough of Manville and Franklin Township.