Draft Appendix T – Chesapeake Bay TMDL for Nutrients and Sediment

T-1 September 24, 2010

Appendix T. 

Susquehanna Dams White Paper

This appendix contains a deliberative document related to the sediment trapping effects of the Susquehanna 

River dams and their effect on the Chesapeake Bay TMDL.
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Assessment of the Susquehanna River Reservoir Trapping 

Capacity and the Potential Effect on the Chesapeake Bay 

Prepared for: United States Environmental Protection Agency

Prepared by: Tetra Tech, Inc., 10306 Eaton Place, Suite 340, Fairfax, VA 22030

1. Introduction

In developing the Chesapeake Bay Total Maximum Daily Load (TMDL), EPA must account for a vast 

array of dynamics that affect the loadings to the Chesapeake Bay and how to appropriately assign load 

allocations to each state. A large influencing factor in sediment and nutrient loads to the Chesapeake Bay 

are the dams along the lower Susquehanna River, which retain large quantities of sediment in their 

reservoirs. The three major dams along the lower Susquehanna River are the Safe Harbor Dam, Holtwood 

Dam, and the Conowingo Dam. This document looks at the dams’ effects on the pollutant loads to the 

Chesapeake Bay and how those loads will change when the dams no longer function to trap sediment. 

2. Sediment Trapping and Storage Capacity

Annually, the reservoir system traps approximately 70 percent of the sediment passing through the system

(Langland and Hainly 1997). The trapping capacity is the ability of a reservoir to continue storing 

sediment before reaching an equilibrium, after which the amount of sediment flowing into the reservoir 

equals the amount leaving the reservoir, and the stored volume of sediment is relatively static. The 

sediment storage capacity is the actual maximum amount of sediment that can be stored in a reservoir 

when it is at equilibrium. 

Safe Harbor Dam (Lake Clarke) and Holtwood Dam (Lake Aldred)

Lake Clarke and Lake Aldred have no remaining sediment trapping capacity. The two lakes have been in 

long-term equilibrium for 50 years or more. 

Conowingo Dam and Reservoir

The Conowingo Reservoir is divided into three parts: upper, middle and lower. The upper and middle 

portions of the reservoir are in long-term equilibrium. Other than temporary increases in sediment storage 

due to scour events, there is no remaining storage capacity (Langland 2009a). 

The lower part of the reservoir is the final 4 miles from just above Broad Creek to the Conowingo Dam. 

Between 1996 and 2008, 12,000,000 tons of sediment were deposited in the Conowingo Reservoir, 

primarily in the lower part (Langland 2009a). The total amount of sediment stored in the lower part of the 

reservoir was 103,000,000 tons by 2008 (Langland 2009a). The lower part of the Conowingo Reservoir is 

the only section of the entire three-reservoir system that has not reached long-term sediment storage 

equilibrium. Some trapping capacity remains in this portion of the reservoir. 

3. Expected Time Remaining until Sediment Storage Capacity Is Reached

The sediment storage capacity of Conowingo Reservoir has been decreasing since 1929, except during 

temporary scour events, such as the one during the Big Melt in January 2006 (Langland 2009a). The 

average reservoir sediment-deposition rate from 1959 to 2008 was 2,000,000 tons per year (Langland 

2009). The long-term trapping efficiency of the Conowingo Reservoir has remained relatively stable at 

around 55 percent for the last 30 years (Michael Langland, USGS, personal communication, November 4, 

2009). 
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According to the U.S. Geological Survey’s (USGS’s) most recent study, 20,000 acre-feet of sediment 

storage remain in the Conowingo Reservoir from Hennery Island to the dam; this translates to 30,000,000 

tons of sediment (Langland 2009a). Given the rate of transport is 3,000,000 tons per year, and the rate of 

deposition is 2,000,000 tons per year, if there are no major scouring events in the Conowingo Reservoir 

and the sediment input does not change, the remaining capacity will be filled in 15–20 years (Langland 

2009a). Once the sediment storage capacity is reached, sediment loads transported downstream past the 

reservoir will approach the loads transported from upstream (Langland 2009a). 

However, because Langland notes that the time until the reservoir reaches capacity is affected by three 

factors—sediment transport into the reservoir, scour removal events, and sediment trapping efficiency—

the time until steady state conditions are reached could be extended to 25–30 years (Langland 2009b). That 

assumes sediment transport decreases from 3.2 to 2.5 million tons/year, statistically expected scour events 

occur, and the long-term trapping efficiency remains at 55 percent (Langland 2009b).

It should be noted that the sediment trapping efficiency of the reservoir is highly variable, depending on 

rainfall. During drought conditions, the trapping efficiency can increase to 85 percent, and during wet 

periods, the trapping efficiency can fall to 40 percent (Michael Langland, USGS, personal communication 

January 15, 2010). 

4. Effects on Chesapeake Bay Once Sediment Storage Capacity is Reached

As of 1997 the Susquehanna River contributed roughly 50 percent of the fresh water discharge to the 

Chesapeake Bay and about 66 percent of the annual nitrogen load, 40 percent of the phosphorus load, and 

25 percent of the suspended sediment load from non-tidal parts of the Bay (Langland and Hainly 1997).

According to USGS water quality sampling in 1985–1989, pollutant loads in the Susquehanna River 

increase substantially below Harrisburg, Pennsylvania: total nitrogen increased 42 percent, total 

phosphorus increased 49 percent, and total suspended sediment increased 50 percent compared to loads at 

Harrisburg (Reed et al. 1997). The increased load is a result of more urbanized areas, agrochemical 

fertilizers and manure, and fewer forested areas (Reed et al. 1997). A significant percentage of those 

pollutant loads are captured by sediment deposition behind the dams, primarily the Conowingo Dam.

Once the Conowingo Reservoir reaches the sediment trapping capacity, the sediment and nutrient loads 

delivered to the Chesapeake Bay via the Susquehanna River will equal the load delivered into the reservoir 

system (Langland and Cronin 2003). Once storage capacity is reached, the nitrogen load will increase by 2 

percent; the phosphorus load will increase by 40 percent; and the suspended sediment load will increase by 

at least 150 percent (Langland and Cronin 2003). 

5. Proposed Activities to Address Sediment Build up Behind the Dam

Dredging

The Susquehanna River Basin Commission Sediment Task Force examined the issue of finding options to 

address the sediment accumulation behind the Conowingo Dam and concluded that dredging may provide 

the needed sediment storage capacity behind the dams (SRBC 2002). 

In 2009 the U.S. Army Corps of Engineers (USACE) Baltimore District received funds to conduct a study 

of sediment management in the Conowingo Reservoir. The investigation could be developed as a 

Sediment Management Plan, to prioritize areas for work and make recommendations to implement 

sediment reduction options (Compton 2009). The study approach outlined by the USACE is conceptual, 

and the final components will be determined with input from the cost-share sponsor. The USACE has not 

yet found a cost-share partner for this feasibility study (Anna Compton, USACE Baltimore District, 

personal communication, December 22, 2009). 
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Conowingo Hydroelectric Project Relicensing Process

The Conowingo Hydroelectric Project is undergoing relicensing. On February 4, 2010 FERC (Federal 

Energy Regulatory Commission) accepted Exelon’s Revised Study Plan, including the requested study 

Sediment Introduction and Transport (Sediment and Nutrient Loading) which will address ―the effects of 

the Conowingo Project and its operation on upstream sediment and nutrient accumulation, sediment 

transport past the project, and sediment deposition and distribution upstream and downstream of the 

projects‖ (Exelon Corporation 2009). Specific tasks include a review of existing information regarding 

sediment and nutrient storage capacity, accumulation rates, scouring events, and such, in the Conowingo 

Reservoir; an analysis of the effects of project operations on habitat and substrate below the dam; and a 

review of watershed-based management efforts and load reduction successes. Exelon noted that the 

―estimated cost in 1995 dollars of dredging to simply keep up with annual sediment inflow (estimated to 

be 2.3 million cubic yards per year at the time) was $28 million per year. Using Means Cost Indices the 

comparable 2009 cost would be $48.44 million. 

Cost Comparison of Dredging and Other Nutrient and Sediment Reduction Strategies 

Comparisons with cost estimates for dredging Baltimore Harbor and Channels from the Dredged Material 

Management Plan and Final Tiered Environmental Impact Statement (Weston Solutions 2005) reveal that 

dredging costs are highly variable, and, to a large extent, depend on the selected destination and use of the 

dredged materials. Costs can be as little as $12/yd3for artificial island creation or beach nourishment and 

as much as $69/yd3if dredged materials are taken to a confined disposal facility (Weston Solutions 2005). 

The sediment management feasibility study proposed by the USACE, and awaiting a cost-share sponsor, is 

likely the best mechanism to determine the true cost of dredging the Conowingo Reservoir.

Cost-Effective Strategies for the Bay (Chesapeake Bay Commission 2004) outlines the six most costeffective practices to reduce nutrient and sediment loading to the Chesapeake Bay. Table 1 summarizes 

the six selected practices and their estimated costs and compares them to the estimated costs of dredging 

the Conowingo Reservoir. Rough calculations of dredging costs at Conowingo were based on the cost 

assumptions used by Exelon and SRBC and the assumption that 1 yd3of sediment weighs 0.945 tons.
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Table 1. Cost-Effective Strategies for Reducing Nitrogen and Sediment Loads to the Bay Compared to Estimated Dredging 

Costs

Practice

Annual nitrogen 

reduction at maximum 

feasible level of 

implementation

Annual phosphorus 

reduction at maximum 

feasible level of 

implementation

Annual sediment 

reduction at maximum 

feasible level of 

implementation

Wastewater Treatment 

Plant Upgrades

35 million lbs @ 

$8.56/lb

3 million lbs @ 

$74.00/lb

Not applicable

Diet and Feed 

Adjustments

Under development 0.22 million lbs @ no 

additional cost (poultry 

only)

Not applicable

Traditional Nutrient 

Management

13.6 million lbs @ 

$1.66/lb

0.8 million lbs @ 

$28.26/lb

Not applicable

Enhanced Nutrient 

Management

23.7 million lbs @ 

$4.41/lb

0.8 million lbs @ 

$95.79/lb

Not applicable

Conservation Tillage 12.0 million lbs @ 

$1.57/lb

2.59 million lbs @ no 

additional cost

1.68 million tons @ no 

additional cost

Cover Crops 23.3 million lbs @ 

$3.13/lb

0.44 million lbs @ no 

additional cost

0.22 million tons @ no 

additional cost

Rough calculations of 

dredging costs

Annual nitrogen 

dredged based on 

removal equal to annual 

trapped amount

Annual phosphorus 

dredged based on 

removal equal to annual 

trapped amount

Annual sediment 

dredged based on 

removal equal to annual 

trapped amount

Dredge Conowingo 

Reservoir

3 million lbs @ 

$16.42/lb

3.48 million lbs @ 

$14.15/lb

4,420 million lbs @ 

$0.01/lb

Source: CBC 2004

6. Proposal for Addressing the Sediment and Phosphorus Load in the 

Chesapeake Bay TMDL

EPA’s intention is to assume the current trapping capacity will continue through the planning horizon for 

the TMDL (through 2025). The Conowingo Reservoir is anticipated to reach a steady state in 15 – 30 

years, depending on future loading rates, scour events and trapping efficiency. The steady state condition 

is at the limits of the planning horizon for the TMDLs and, depending on conditions, could be well beyond 

the planning horizon. 

Under these assumptions, the wasteload allocations (WLA) and load allocations (LA) would be based on 

the current conditions at the dam. This represents a business-as-usual scenario in which the future 

diminished trapping capacity behind the Conowingo Dam is not considered in developing of the wasteload 

WLA and LA. 

If future monitoring shows the trapping capacity of the dam is reduced, then EPA would consider adjusting 

the Pennsylvania, Maryland and New York 2-year milestone loads based on the new delivered loads. The 

adjusted loads would be compared to the 2-year milestone commitments to determine if the states are 

meeting their target load obligations. 

Future increases in sediment and phosphorus downstream of the dam can be minimized by making 

implementation activities above the dam a management priority. This will decrease the overall loads of 

sediment and phosphorus, and extend the time until trapping capacity is reached. The states should work 

together to develop an implementation strategy for the Conowingo Dam and take the opportunity to work 

with FERC during the relicensing process for Conowingo Dam. 
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7. References

Chesapeake Bay Commission. 2004. Cost-Effective Strategies for the Bay: 6 Smart Investments for 

Nutrient and Sediment Reduction. Chesapeake Bay Commission, Annapolis, MD. 

Compton, A. 2009. Sediment behind the Dams. Presented at the Susquehanna River Basin Commission 

Sediment Task Force Meeting, October 29, 2009. USACE, Baltimore, MD.

Exelon Corporation. 2009. Conowingo Hydroelectric Project, FERC Project No. 405; Filing of Revised 

Study Plan. Prepared by Exelon Corporation, Washington, DC, for the Federal Energy Regulatory 

Commission, Washington, DC.

Langland, M.J. 2009a. Bathymetry and Sediment-Storage Capacity Change in Three Reservoirs on the 

Lower Susquehanna River, 1996-2008. U.S Geological Survey Scientific Investigations Report

2009-5110. U.S. Geological Survey, Reston, VA.

Langland, M.J. 2009b. Lower Susquehanna River Reservoir System Bathymetry. Presented at the 

Susquehanna River Basin Commission Sediment Task Force Meeting, October 29, 2009, 

Baltimore, MD.

Langland, M., and T. Cronin, eds. 2003. A Summary Report of Sediment Processes in Chesapeake Bay and 

Watershed. U.S. Geological Survey Water-Resources Investigations Report 03-4123. U.S. 

Geological Survey, New Cumberland, PA.

Langland, M.J., and R.A. Hainly. 1997. Changes in Bottom-Surface Elevations in Three Reservoirs on the 

Lower Susquehanna River, Pennsylvania and Maryland, Following the January 1996 Flood—

Implications for Nutrient and Sediment Loads to Chesapeake Bay. U.S. Geological Survey Water 

Resources Investigations Report 97-4138. U.S. Geological Survey, Washington, DC.

Reed, L.A., C.S. Takita, and G. Barton. 1997. Loads and Yields of Nutrients and Suspended Sediment in 

the Susquehanna River Basin, 1985-1989. U.S. Geological Survey Water Resources Investigations 

Report 96-4099. U.S. Geological Survey, Lemoyne, PA.

Seay, E.E. 1995. Managing Sediment and Nutrients in the Susquehanna River Basin. Publication 164. 

Susquehanna River Basin Commission, Harrisburg, PA.

SRBC (Susquehanna River Basin Commission). 2002. Sediment Task Force Recommendations. 

Publication 221. Susquehanna River Basin Commission, Harrisburg, PA.

Weston Solutions. 2005. Baltimore Harbor and Channels (MD and VA) Dredged Material Management 

Plan and Final Tiered Environmental Impact Statement. Prepared by Weston Solutions, West 

Chester, PA, for the U.S. Army Corps of Engineers, Baltimore District, Baltimore, MD.