On Sept. 30, I watched the streets of my neighborhood become a river system. That evening, as I watched the local news, I was struck by a reporter’s comment about what happened with the flooded streets of Portland midday.

Streets everywhere became hazardous to traverse with the incredible quantity of rain falling on them. Streets closer to sea level had the added burden of runoff rushing down from higher elevations and water gushing up through manhole covers.

Worse, areas of high water were unable to drain because of both the quantity of water overwhelming drainage systems and the astronomical high tide.

What struck me was when the reporter said the streets had finally become safe to travel because the rain had been sucked down the storm drains, making our streets passable once again.

Right he was. Yet where did it all go?

It went straight into Casco Bay.

Billions of gallons. Gallons of “anything but fresh” rainwater runoff – rainwater mixed with sewage overflows, any air pollutants that adhere to water droplets and every toxic component that could possibly wash off our roofs, lawns, driveways, gardens, parking lots, sidewalks, roads and byways, especially substances that tend to adhere to sediments – which washed off by the ton.

This runoff would change the color of Casco Bay for days, from blue to ghastly. Following one of these massive rain events, we’ve seen the bay turn brown, brownish yellow, brownish green – anything but blue, with anything-but-fresh rainwater sitting on it, mixing with it, polluting it.

The quantity of polluted water following one of these storms is hard to wrap your head around. The watershed – the land area that drains into Casco Bay – covers 985 square miles. According to the U.S. Geological Survey, 1 inch of rain falling over 1 square mile can result in 17 million gallons of water.

If, for the moment, we assume that the amount of rain Portland received was the same across the watershed, we could estimate that over 96 billion gallons landed on our landscapes – it’s tough to understand how much water that is.

Some of this water seeped into parched soils; when the soils became saturated, the rest of the water went running off into streams, waterways, roadways and eventually down storm drains.

All at once. Loaded with pollutants. What a shock to the ecosystem of Casco Bay.

We are likely to see more extreme weather, part of an escalating pattern of alarming precipitation events.

As the organization charged with protecting the health of Casco Bay, we took a look at what happens to our coastal waters immediately after large rain events, measuring the acute impact as it happens.

On a beautiful July day in 2014, following a string of gorgeous sunny days with no rain, we went out near Fort Gorges and took some water samples. We sent the samples to a laboratory.

The salinity was what we would expect at 30 parts per million. Total suspended solids were 6.1 milligrams per liter, and bacteria were found, though not in measurable quantities.

A few weeks later, we went out to Fort Gorges and sampled again, on the tail end of the historic Aug. 13 storm when 6 inches of rain fell in Portland – a storm very much like the one we just experienced Sept. 30.

This time, as we lowered our instruments, we were stunned to find that we could not detect ocean salinity until our instruments were 18 feet down. That was a lot of runoff sitting on top of the bay.

Total suspended solids were 10 times what we found on the dry day. Bacteria levels were 170 colony-forming units per 100 milliliters of water; the acceptable risk for recreational contact, such as swimming, is 126 units.

There is an old and misleading adage, “the solution to pollution is dilution” – a false notion, given the levels of contamination we found in vast amounts of runoff.

The bay is the feature that makes this region unique. Casco Bay belongs to all of us. Join us in thinking twice about simple actions that can have far-reaching consequences.

Please be cautious about anything you do that leaves a residue anywhere that could end up in the bay, from what you wash down the kitchen and bathroom sink drains to what might wash off your property.

Thank you for helping us take care of the health of the bay. Find out what you can do at cascobay.org.