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How rainstorms affect the Bay

Today’s rain reminds us that heavy rainstorms can deliver a significant insult to the health of Casco Bay.

Rainwater runoff resulting from intense storms flows into the Bay, bringing with it a host of pollutants including nitrogen, pesticides, oil, and heavy metals. If rainfall is heavy enough, the large dose of freshwater can temporarily lower the salinity, or saltiness, of the Bay.

This year we experienced a long stretch of dry weather through May and into June, which was followed by almost three inches of rain in just two days in late June. This combination created conditions that brought a large amount of freshwater into the Bay through both runoff and increased river flow. The river flow increase can be seen in the United States Geological Survey river gauge data from the Royal River. The Royal empties into Casco Bay near our Continuous Monitoring Station.

The graph above compares the flow of water in the Royal to the salinity at our Station in Yarmouth. The increase in fresh water following the storm causes a decrease in salinity at the station. It takes significant amounts of freshwater to dilute the water in the Bay. This suggests large amounts of stormwater runoff deliver pollutants to the Bay.

The increased river flow was still visible into mid-July. While out conducting our Seasonal Water Quality Monitoring on July 15, Staff Scientist Mike Doan and Casco Baykeeper Ivy Frignoca measured salinity values of 6.4 parts per thousand (ppt) at the mouth of the river where values are typically around 29.0 or 30.0 ppt. As they moved away from the river mouth and out toward deeper water, values were still lower than usual at 12 to 15 ppt.

In the period of time following the storm, we saw a dramatic increase in blooms of nuisance algae. This is potentially the result of increased nutrients in the stormwater runoff, as well as high water temperatures.

As Casco Bay continues to get warmer and we experience more frequent and intense rain storms because of climate change, we may see more pollutants and more nuisance algal blooms in the Bay.

This is one of the many reasons why Friends of Casco Bay launched the Climate Change and Casco Bay Fund, which will help us put two more Continuous Monitoring Stations in the Bay, one near Portland and one near Harpswell, and operate all three stations for ten years. The $1.5 million Climate Change and Casco Bay Fund will be used over the next decade to understand the ways in which our waters are changing, while we engage the community in assessing and adapting to climate change. You can read more about the Fund and our 10-year plan here.

What can you do about stormwater pollution?

  • Did you know that the fertilizers and pesticides you put on your lawn may end up in Casco Bay and contribute to these problems?
  • Keep pollutants from entering the Bay by reducing or eliminating the fertilizers and pesticides you apply to your lawn.
  • Become a Water Reporter. Our volunteer observing network tracks the spread of algal blooms around Casco Bay. We use that information to alert the State to changes in our coastal waters.
  • Make a donation to support the Climate Change and Casco Bay Fund.