It's report card time! The U.S. Environmental Protection Agency (EPA) gives the Charles River a letter grade for overall cleanliness every year.

This year's grade: B. It's a dip from last year's A-, but not too different from other recent years. (Grades from the last 24 years are listed at the bottom of this Q&A.)

What happened this past year? To understand the report card and the results, we talked with EPA Region 1 Acting Administrator Deborah Szaro and Charles River Watershed Association Executive Director Emily Norton. Here is a lightly edited version of the conversation:

How is the report card grade determined?

Szaro: For the last 24 years, we’ve been working with the Charles River Watershed Association to produce the report card. They collect monthly water quality samples at 10 monitoring sites from the Watertown Dam to Boston Harbor, and the EPA uses this data to calculate a letter grade.

An "A" means the water almost always met standards for boating and swimming; a "B" means it met standards for almost all boating and some swimming; a "C" means it met standards for some boating and some swimming; a "D" means it met standards for some boating but no swimming; and an "F" means it did not meet standards for boating or swimming.

Why was this year’s grade lower than last year? And were you surprised?

Norton: We are disappointed in the drop, but we are not surprised. When it comes to the Charles, a lot depends on the weather. How much rain have we had? How much stormwater runoff have we had? We had a lot of wet weather in 2018.

Szaro: Massachusetts saw record rainfall in 2018. When it rains, the water picks up pollutants as it runs off our urban streets, our parking lots, our rooftops and carries it to rivers either directly or via storm drains.

Does climate change factor in?

Norton: Climate change is a big threat to the Charles River because stormwater is a big threat to the Charles River. Climate scientists predict that the Northeast will get more heavy rainfalls that will bring pollutants into our waterways. In fact, 2019 has already been very rainy, so I imagine we’ll see similar results next year.

Are there other problems beyond stormwater?

Norton: There are plenty of challenges to our water bodies in Massachusetts, but stormwater is a really, really big one. It’s certainly the biggest threat to the Charles River. It carries nutrients from gasoline, fertilizer, trash, metals and other toxins over impermeable surfaces right into the water.

A lot of the work we’re doing now is with communities to help reduce the amount of stormwater runoff entering the Charles. People should keep in mind that stormwater that goes into the stormwater system goes straight into the Charles; it is not treated like wastewater. That blue-green algae that you see sometimes, it’s really, really dangerous and toxic.

What do the water quality tests measure?

Szaro: The tests look for various bacteria, including E. coli, as well as dissolved oxygen levels, chlorophyll, pH level and many other things.

Stormwater pollution bring nutrients like phosphorus into our waterways, which can cause algal blooms. Sometimes these blooms contain toxic cyanobacteria that can be quite harmful to humans or pets that come into contact with it.

The tricky thing is, we don’t know if an algae bloom is toxic or not until we test it, and blooms often move quickly down the river so it’s hard to keep tabs on them. Our best bet is to keep reducing the amount of phosphorous that’s getting into the river.

How do we do that?

Szaro: In order to discharge stormwater into a Massachusetts waterway, municipalities need a Municipal Separate Storm Sewer System permit, or MS4.

The EPA has been working to upgrade the permit, and last July, those changes went into effect. The new permit gives better direction to cities and towns about how to reduce stormwater runoff, and how to ensure the stormwater that does enter our waterways contains less pollution.

Some of the changes are what I would call good housekeeping measures — better street sweeping practices and more regular catch basin cleaning. But the permit also targets phosphorus runoff, and some areas near the Charles must reduce this form of pollution by about half.

Another problem this permit addresses is illicit sewer-stormwater connections. Some buildings have a connected sewer and stormwater system, which can dump a lot of unhealthy things into our waterways. We’re constantly working with cities and towns to both identify and remove existing connections, as well as prevent them from happening during the construction process. (We also have a specific program to address combined sewer overflows, which is a separate, but related issue.)

Bottom line: Is it safe to swim or boat in the Charles?

Szaro: We do feel the public can safely boat on the Charles almost all of the time. Sometimes it’s even safe to swim. Boathouses along the Charles use a flag system to let you know when the water is safe for boating. (You can also check on the CWRA website.)

As for swimming, that's a little more complicated. Even if the water is clean enough to swim in, in general, swimming is prohibited in the Charles River. (The exception is during sanctioned events — like this weekend's City Splash Event.)

Norton: A lot of the time the water isn't safe enough to swim in, but we hope to change that. We would love for people to be able to safely swim in the Charles. That said, I think the story of the cleanup of the Charles — even though it’s a B grade this year and not an A- or an A — is a success story.

(According to an EPA press release: "In 2018 during dry weather, 94% of the Charles River samples met the state’s bacterial water quality standards for boating, and 66% of the samples met the state’s criteria for swimming. In wet weather, the percentage dropped slightly for boating to 91% and dropped more significantly for swimming to 47%.")

Here are the report card results going back to 1995, the first year a grade was given:

2018 B

2017 A-

2016 B

2015 B+

2014 B+

2013 A-

2012 B+

2011 B

2010 B+

2009 B+

2008 B+

2007 B++

2006 B+

2005 B+

2004 B+

2003 B-

2002 B

2001 B

2000 B

1999 B-

1998 C+

1997 C

1996 C-

1995 D

The grade is based on a comparison with previous years and general trends. Given that the grades are based on a limited set of data, some annual fluctuations depending on weather are expected. Figures represent the percentage of time that state bacterial standards are met.