Conowingo Study Points to Problems

One of the recent controversies in the arguments over the Chesapeake Bay "Bay Diet", has been over the importance of Conowingo Dam as a source of pollutants. In particular, farmers point to Conowingo as a problem of greater magnitude and importance than agricultural nutrients, and say that the Bay Diet unnecessarily restricts them, while ignoring inputs that come from behind the dam, and that fixing the dam problem (pun intended) should be addressed before further restrictions on agriculture. A new study may help them make that case:

Study says more nutrient reductions may be needed to offset filling of Conowingo Dam

A portion of the sediment washing down the Bay’s largest tributary — and greatest source of pollution — has been trapped in the dam’s 14-mile reservoir since it was constructed in 1928-29. Recent studies warned that the amount of pollution reaching the Bay would increase when the reservoir was filled, setting back Chesapeake cleanup efforts.

The new Lower Susquehanna River Watershed Assessment draft report says that day has arrived, and the reservoir is in a state of “dynamic equilibrium.” That means the sand, silt and clay washed down the Susquehanna may accumulate behind the dam in some years, only to be flushed out during severe storms that are likely to hit every few years.

So the dam no longer serves as a long term sink for nutrients, but rather as a kind of pollution injector, adding pollution to the Bay above the "normal" baseline during severe storms:

In effect, the dam helps to improve downstream water quality during dry periods, but increases pollution during heavy storms when built-up material is “scoured” from behind the reservoir.

One surprise from the study, officials said, was that the nutrients associated with the stored sediment — nitrogen and phosphorus — pose a greater threat to Bay water quality than the sediment. Nutrients spur algae blooms which block sunlight needed by important underwater plants and, when they die, the algae decompose in a process that draws oxygen from the water, contributing to so called “dead zones.”

The additional nutrients associated with the sediment would likely keep some deep-water areas of the upper Bay from meeting Bay cleanup goals for dissolved oxygen, according to the study.

“The water clarity effects of sediment essentially decline once the particles settle,” said Anna Compton, a biologist with the Corps who was the study director. “However, nutrient pollution has a lingering effect that leads to algae blooms and dead zones that have the potential to suffocate and stress aquatic life.”

In effect, they are admitting the farmers complaint that the nutrients from behind the dam are a severe problem, and in particular, make bad years worse.  So why do they come to exactly the opposite conclusion?

The reservoir behind Conowingo Dam has filled, which likely means states will need to make additional nutrient and sediment reductions if Bay cleanup goals are to be met, according to a draft study being released today.

The $1.4 million study, released by the Army Corps of Engineers and the Maryland Department of the Environment, also concluded that dredging built-up sediment from behind the 100-foot-high Susquehanna River dam would have huge costs and provide little benefit.

State and federal officials briefed the media on the findings of the much-anticipated three-year study Wednesday. The future sediment trapping capacity of the dam, located just 10 miles upstream from the Bay, has been a focal point of controversy in recent years.

In my opinion, it's because they can make farmers either cut pollution (and cost farmers money) or force the farmers out of business (and not pollute at all) at essentially no cost to the government. However, the government would have to provide most of the costs of cleaning the pool behind the dam. They could try to force Exelon, current owner of the dam, to bear the cost, but they run the risk of driving the dam out of the power business, and still not getting the required clean up. The damn dam has provided the Bay a net service since 1928; why make life more difficult for the company that provides that service?