The document summarizes bathymetric surveys of three reservoirs along the Lower Susquehanna River: Lake Clarke, Lake Aldred, and Conowingo Reservoir. It finds that the reservoirs are filling with sediment and nutrients at rates that will eliminate their storage capacity within 15-20 years. Recent surveys from 1996-2008 found that 14.7 million tons of sediment was deposited, leaving around 30 million tons of remaining storage capacity in Conowingo Reservoir. Under current conditions, total storage capacity will be reached in 15-20 years, but changes like reduced sediment transport could extend that to 25-30 years. The document recommends more frequent bathymetric monitoring and working with partners to consider solutions like sediment removal or reducing transport from source

1. Lower Susquehanna River Reservoir System Bathymetry In cooperation with - Pennsylvania Dept. of Environmental Protection - Susquehanna River Basin Commission - Lower Susquehanna River Keeper

3. Approximately 55-60% of the sediment and 40% of the phosphorus is being trapped

4. Revised estimate in 2001 of 25-30 years of remaining sediment storage capacity (SSC)

6. Provide updated estimates of remaining capacity for sediment storage

10. Courtesy Exelon Corp.

13. Current “trends” in sediment loads Long-term AVG (1980-1999) (3.2 Mt in, 1.2 Mt out, 55% trap w/scour) From 2000-2008 (avg 3.1 Mt in, 1.2 Mt out, 55% trap w/scour)

14. Estimated Sediment Loads to Reservoirs 1900-2008

15. Streamflow Hydrograph 01578310 - Susquehanna River at Conowingo, 1996-2008 Hurricane Ivan 390,000 cfs (scour threshold)

16. Predicted Sediment Scour vs. Flow 30 Scour estimates 95th percentile 25 tons) 95th percentile Expon. (Scour estimates) 1972 20 15 Predicted scour (millions of 1996 10 2004 Ivan 5 1975 0 300 400 600 700 800 900 1,000 1,100 1,200 500 Instantaneous Flow (times 1,000)

17. Wendy McPherson (USGS) - 9/20/04

18. Hurricane Ivan – September 2004 Susquehanna River Bay Bridge

19. Delta (sand) Density current (Silts) Fine sediments (silts & clays) Idealized Reservoir Sediment Dynamics Turbid inflow Floating Debris Water Surface Relatively clear water (clay) Sluiceway

20. CurrentBathymetry 2008 Safe Harbor Dam (Lake Clarke)

22. Total 92 M tons

25. Total 14 M tons

26. Deepest areas near sides of natural narrow channels (hydraulic scour areas)

29. Total 174 M tons

30. Total storage 204 M tons (SSC)

31. Approx 30 Mt remain

33. Conowingo 3-D Longitudinal Profile Distancebelow surface, in feet Pa/Md state line 0 5 10 15 20 25 30 35 40 Distance upstream (feet times 10,000)

34. Loss of sediment-storage capacity with time

35. SSC Predictions From 1996-2008, 14.7M tons deposited Approximately 30M tons remain before SSC Predictions at steady state -- 15-20 years Predictions with future change - decrease transport (3.2 to 2.5), add 5 years - statistically expected scour removal, add 5 years - trapping efficiency of 55%, add 0 years 25-30 years with future change

37. Less scour producing events resulting in slightly lower sediment transport to upper BAY

38. Remaining capacity of 30M tons

39. Under current conditions, 15-20 years to SSC

40. Could be extended to 25-30 years considering future scenarios

42. USACE new study

43. Reconvene the Sediment Task force (STF)

44. Some solutions under consideration include (from STF) - reservoir sediment removal - reduce sediment transport from source areas (BMP’s stream stabilization, wetlands, buffers, etc.) - use of floodplains to dissipate stream energy and store sediment

45. The ultimate DESTINY of all reservoirs is to fill with sediment - Linsley and others, 1992 Thank You