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High Water on the Susquehanna

Causes of high water on the Bay

This is in response to James P Miller, who asks the question: "Every spring and fall when the water is high the question always comes up. How many flood gates are there? Someone should go and count the flood gates and the open spill ways so this discussion can be settled."

The answer is too lengthy for the forum, so I have had to make a blog of it. I would appreciate input from people closer to this nature than myself.

I am not a climatologist, but I can state authoritatively, and you can check this out at http://en.wikipedia.org/wiki/Conowingo_Dam, there are 53 gates. I have to check with Exelon every time there is a weather incident.

High water does not just come in the spring and fall; there are many reasons why the water may be exceptionally high:

Spring Tides: These occur approximately twice a month, and, depending on the proximity of the moon to the earth, can raise the water about 4 feet over the norm. (We DID have a spring tide this week.)

Winds: Strong winds from the south or southeast can push water up the bay, as in fact happened yesterday, on top of a spring tide.

Excessive Rainfall: The Susquehanna watershed covers many thousands of square miles, stretching from Northern New York State, and including New Jersey, Delaware, Pennsylvania, and Maryland. A storm over any of these areas will increase the load on the dam; if, as happened this week, all these states are affected, then there is potential for disaster.

The direction of the storm can seriously affect this. If the storm moves from North to South, then the effect can be dramatic. The Susquehanna River is 464 miles long, originating not far south of Lake Ontario. There are 5 dams in New York, 16 dams in Pennsylvania, and one in Maryland. Not all of these dams are for producing power. If the northernmost dam has to open their floodgates, then there is a surge on the next dam. If that area is also experiencing heavy rainfall, then the surge from the first dam has to be added to the local runoff. Each dam waits until the latest moment, in terms of local safety, before they open their gates. Each dam notifies the next dam downstream before doing this. The Conowingo Dam is the last in line of this process, and has to accept all the other dams’ overages, plus any runoff from Pennsylvania and Maryland.

If, however, as happened this week, the storm does not come from the north, then the situation is considerably relieved. This week Maryland, and then Delaware and Pennsylvania saw the first of the rainfall. This allowed Exelon to monitor the situation, and determine when, and how many gates, they needed to open. Exelon are very aware of the effect of opening ANY gates in the event of a storm situation: this can have devastating effects on the Chesapeake Bay, if, as you look at the color of the water tomorrow, you can observe for yourself. This isn’t just a Maryland problem, every state where rivers, streams or runoff go into the bay contribute to this problem.

Ice Melt: If the Pocono Mountains in Pennsylvania have a harsh winter, they can experience many feet of snow. In the spring, a sudden and dramatic change of climate can force this snow to melt very quickly, filling rivers leading to the Susquehanna.  Generally, this “fast melt” can be beneficial to the bay, as it is mainly melted pure water, and therefore does not carry the nitrates and other harmful materials produced by a normal runoff. This has a beneficial effect on the quality of water in the bay.


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Dave in BR November 01, 2012 at 05:15 PM
John, very informative write-up. Thank you! Do you know if Exelon ever opens the gates in anticipation of a storm and the potential for rising waters in an attempt to mitigate the need for opening many gates during or after rising waters?
John Cole November 02, 2012 at 06:48 AM
I believe the answer to your question is "No". The dam needs to maintain a minimum level to continue the production of electricity. As soon as the water reaches a certain height they begin to open gates, to maintain only what is required for their production. The problems the dam experiences are mainly caused by upstream dams, one at a time, opening their gates to avoid flooding or breach. This has a cascading effect, and the water level rises extremely quickly in the catchment north of the dam.


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