Why You Should Avoid Swimming In Any Lake After A Heavy Rainfall

Few things sing a siren song as enticing as a cool lake on a scorcher of a day. It's enough to make you strip off your hiking gear and dive right in. However, at the risk of vacation-ending digestive distress or worse, make sure you always avoid lake water after a heavy downpour or a big storm at all costs.

When considering the most dangerous spots to swim in the world, players like sharks, alligators, and rip currents often dominate news headlines. However, unseen germs present a far more pressing danger. Since 1948, more than 50% of waterborne disease outbreaks in the U.S. have occurred after a significant downpour. Similarly, this also leads to gastrointestinal sickness in almost 100 million people every year, according to a 2003 study published in the American Journal of Public Health. 

As stormwater floods into lakes, rivers, and streams, it carries with it a medley of contaminants — from agricultural waste like pesticides, chemicals, and fertilizers to urban runoff, human and animal feces, and heavy metals like lead and mercury. Additionally, home sewers and wastewater treatment plants often aren't built for huge deluges, causing water contaminated by Salmonella or harmful E. coli to overflow into places people go.  According to the U.S. Department of Health and Human Services, these bacteria can be found in both animal and human waste, and lead to symptoms such as diarrhea, intense stomach discomfort, and even death. It can also harbor dangerous organisms, like brain-eating amoeba, which can lead to fatal infections if water enters the nose. Cuts or tiny scrapes also serve as entry points for more dangerous bacteria to enter your bloodstream, leading to serious infections. Even potential minor illnesses like skin rashes or eye infections aren't worth it for a quick dip in questionable water.

Although you can't always tell if lake water is clean using the naked eye, there are a few indicators you should look out for that will have you thinking twice about whether or not to jump in. First, look for any warning signs (literally) about potential contaminants. These can usually be found in high-traffic areas — like the entrance to the park, near the bathrooms — or along a walkway every visitor has to pass. However, the reality is that lakes often lack the signage you'd typically find at other bodies of water because there are multiple entry points, which makes it difficult to monitor and post these warnings.

To be on the safe side, you'll also want to check the U.S. Environmental Protection Agency, or a water quality monitoring group specific to your area, for any water safety advisories before traveling to any nearby lakes. Nonetheless, even if a local agency determines the water is safe, it's usually best to postpone your trip until at least 48 hours after a big rain. This, because, in the event of a recent storm, the reported water quality stats may not accurately represent the current conditions.

You'll also want to postpone your swim if the water looks murky, cloudy, or unusually muddy, or if it gives off a bad smell. These are all signs of both stormwater contamination and harmful algal blooms (HABs), which can produce dangerous toxins. Even if you don't drink or accidentally ingest contaminated water you can still become ill through skin contact. Plus, once you're out of the water make sure to also check for any signs of swimmer's itch — an allergic reaction to microscopic parasites that can cause intense itching and redness.