After reviewing correspondence between the Federal Energy Regulatory Commission (FERC) and Constellation between 2022 and 2025, the record shows a pattern of unresolved instrument failures, recurring structural deterioration and outstanding questions regarding uplift, seepage, and bedrock stability under extreme flood conditions. These findings lead to valid concerns regarding the Conowingo dam’s short and long-term stability and safety, particularly for a dam classified as a “High Hazard” dam, meaning “failure would likely result in loss of human life, extensive property damage to homes and other structures, or cause flooding of major highways such as State roads or interstates”.[1]
Some of the most noteworthy issues include:
1. Piezometer Failures
Piezometers are essential instruments used to measure internal water pressures beneath the dam, providing essential data required for stability analysis.[2] In March 2024, FERC corresponded with Constellation regarding a 2022 Piezometer Assessment Report, stating that only 27 of the 67 interface piezometers were providing reliable data. In FERC’s May 2025 review of a Piezometer Restoration Study, FERC noted that although Constellation had redeveloped 16 failed piezometers after conducting a work and restoration plan, 12 of those 16 piezometers were once again questionable or unreadable within months, including two that initially appeared successful. In the March 2024 letter, FERC had stated that the lack of dependable data results in “lack of confidence” in Constellation’s uplift assumptions and that Constellation’s stability analysis cannot be given a final determination until functional instrumentation is installed and monitored.
For context, uplift pressure occurs when water builds up beneath a dam or other hydraulic structures and pushes upward on its base. Tracking this pressure is essential provided that unchecked uplift can reduce foundation stability.[3] Without reliable uplift pressure measurements, FERC cannot confirm whether the dam’s operational components are performing as assumed in stability models.
View letters here
2. Repeated Structural Deterioration: Cracking, Concrete Damage, Vegetation Growth, and Debris Accumulation
Across four consecutive inspection cycles (2022-2025), FERC identified:
- Vegetation growing through cracks in spillway and structural concrete,
- Concrete deterioration on spillway bays, non-overflow sections and gallery walls,
- Debris accumulation on spillway gates and intakes that could impair operations
- Leakage through the inspection gallery
Although Constellation reported maintenance attempts and the development of debris management plans, these conditions continued to recur, prompting FERC in August 2025 to direct Constellation to prioritize repairs under its Concrete Asset Management Program (CAMP). Furthermore, chronic vegetation and cracking may indicate potential water infiltration and potentially progressive deterioration. Repeated findings and correspondence suggest the issues are not being fully resolved.
View letters here
3. Persistent Seepage and Ponding Behind Powerhouse Units 8-11
Since 2023, FERC inspections have documented ongoing seepage in the confined space behind Units 8-11, ponded water that worsened in the 2025 inspection, and uncertainty regarding the source (FERC suggests possible increased seepage or reduced pumping performance). In 2023, FERC recommended that Constellation conduct a dye-tracer test and additional investigation which have not yet been reported. According to the Army Corps of Engineers, “seepage control is necessary to prevent excessive uplift pressures” among other hazard mitigation measures relating to dam structure and foundation.[4] Seepage occurs gradually and may cause collapse due to gradual weakening of the soil beneath it.[5]
View letter here
4. Bedrock, Scour, and Extreme Flood Stability Concerns Remain Under Evaluation
In its October 3, 2025 letter, FERC reviewed Constellation’s White Paper on foundation rock conditions. FERC agreed that existing data does not show an obvious continuous failure plane, but stated:
- Additional coring and geophysical data are necessary to confirm interpretations
- Scour potential beneath abutments AM-02 to AM-04 requires further evaluation because scour can alter seepage paths and uplift pressures
- Assumptions supporting stability during the Probable Maximum Flood (PMF) still require justification
- FERC questioned whether additional foundation drains may be necessary
- Several model inputs, assumptions, and computational methods require clarification and sensitivity analysis.
In this letter, FERC states that it does not accept Constellation’s stability evaluation under PMF-level loading.
View letter here
Additionally, Maryland Department of the Environment recently completed a two-year Probable Maximum Precipitation (PMP) study finalized in early 2025, which will update the hydrologic inputs used for dam-safety design storms statewide. The study was conducted by Applied Weather Associates and relied on storm-based deterministic methodology consistent with NOAA hydrometeorological guidance. The study cites a 1965 Susquehanna River basin report, predating Hurricane Agnes in 1972 as a foundational reference point for predicting PMP levels in Maryland. This reliance on outdated hydrologic data raises questions about whether current PMF modeling fully reflects modern extreme-storm behavior and climate-driven precipitation intensification. These uncertainties further underscore FERC’s conclusion that Constellation’s PMF-stability evaluation cannot yet be accepted and requires additional justification and refinement.
[1] Maryland Department of the Environment. Hazard Classification of Dams. Maryland Dam Safety Program. Hazard Classification of Dams
[2] Gonzalez, Noe. “Piezometers: An essential component to dam safety.” U.S. Army, 6 Aug. 2023, army.mil/article/268859/piezometers_an_essential_component_to_dam_safety
[3] Uplift Pressure.” Fiveable, Fiveable Content Team, Sept. 2025, Uplift pressure Definition – Intro to Civil Engineering Key Term | Fiveable
[4] U.S. Army Corps of Engineers. Seepage Analysis and Control for Dams. Engineer Manual EM 1110-2-1901, Dept.
[5] “Seepage in Geotechnical Engineering.” CEHub, civilengghub.com, 17 Oct. 2025, Seepage in Geotechnical Engineering: A 2025 Complete Guide CEHub