Bob Irvin's commentary ("Maryland's authority to protect the Chesapeake is in jeopardy June 24) is well-meaning but cites the often-stated but misinformed position that the Conowingo Dam contributes to the pollution of the Chesapeake Bay.
None of the sediment and other pollution carried by the Susquehanna River into the bay originates at the dam itself. Rather, all of it has been contributed by the watershed upstream from the dam including (in order of area) nearly half of Pennsylvania and parts of southern New York and northeastern Maryland. In fact, for much of its 80-year history, the dam has acted to reduce the sediment entering the bay by trapping it behind the dam. Of course, the dam basin is now at capacity so the net sediment load flowing over the dam is now equal to the load flowing downriver from above the dam.
It is true that the dam alters the rate of sediment flow depending on the overall water flow rate. During significant storms, the increased Susquehanna River flow rate acts to scour sediment from behind the dam and flow it into the bay. Of course, this is in addition to the increased sediment generated by runoff from the watershed above the dam. So in that sense, the dam does act to amplify the sediment load during these storms. Of course, this also has the effect of freeing up sediment capacity behind the dam so following the storm the sediment load will be reduced below that flowing downriver. Over a sufficiently long period of time, the net effect of the dam on the river sediment load will be zero.
I believe that this was the same conclusion reached by the U.S. Army Corps of Engineers study of the effects of the Conowingo Dam on the Susquehanna River sediment flow into the bay. I also understand that the University of Maryland Center for Environmental Science is starting a two-year study of the sediment load entering the bay from the river, and I have faith that they will reach valid scientific conclusions regarding the effects of the dam.
Given that the Conowingo Dam has no net effect on the overall sediment load entering the bay, it seems inappropriate to demand that the dam owners, Exelon, be responsible for dredging sediment from behind the dam. If dredging is to be done, the cost should be borne by the primary contributors, Pennsylvania and New York.
For the record, I have no connection to any of the organizations involved in this issue and the ongoing debate. I am a private citizen of Maryland whose only interest is in using scientific reasoning to determine the best approach to improve the long-term health of the Chesapeake Bay.
Frederick S. Patt, Chestertown