With Memorial Day behind us, summer has unofficially begun. The Washington DC region has already experienced temps in the nineties, so it was certainly swimming weather this weekend. But weather isn't the only factor that determines whether a day in the water is fun or not. Every year, thousands of people become ill from visiting the beach or a lake, or even from taking a peaceful float down a river. What causes this?

Runoff from urban areas or agricultural regions can carry sewage or animal waste into streams and rivers, which flow into our beloved lakes and the ocean. The risk of this happening is higher after rainfall events, since water that comes down as rain will flow over land and into our waterways, filling up creeks and streams. In many urban areas, sewage treatment plants cannot handle the inflow of water that comes from a heavy rain event. When this happens, the treatment plant will discharge excess waste into surface waters so the sanitation system does not shut down.

Yes, you read that right – raw sewage gets released into our rivers and lakes, on purpose, by the people responsible for keeping our water clean. That waste can carry pathogens that can make us sick. This is of particular concern for those with weakened immune systems, such as the elderly or the very young, as they are more susceptible to illnesses.

Another concern is blue-green algae, also known as cyanobacteria. Excess nutrients entering a pond or lake can cause the growth of this algae. If it is ingested by humans or animals, the algae can cause liver failure – scary stuff!

To avoid getting sick, there are a few precautions you can take. First, take notice of warnings from the health department or posted signs at your favorite swimming hole. If they say it is unsafe to swim, DO YOURSELF A FAVOR AND DO NOT SWIM. Also, avoid swallowing water while swimming no matter where you are, and swim in areas with lots of water circulation, as pathogens and cyanobacteria are less likely to accumulate there.

There is another lesson here – what we do on the land affects our ability to engage in and enjoy the outdoor recreation that we love. By applying fertilizer mindfully (test your soil first, to see if it really needs the nutrients), and by controlling runoff that leaves farm fields and flows through urban areas, we can decrease the likelihood that algae and pathogens will multiply in our swimming spots. Planting trees, building infrastructure that reduces runoff, and using water wisely in our homes are all ways to help this effort. Preventing runoff may help you avoid a summertime illness!

And as always, don’t forget your sunscreen!

The League is on a mission to ensure cleaner water for everyone by monitoring 100,000 more stream sites nationwide by 2022. You can help.

Join the Clean Water Challenge.