That refreshing dip in the lake or a lazy float down the river might be exactly what a hot summer day calls for. But before you wade in, it's worth knowing what could be lurking beneath the surface.

Bacteria, like E. coli, and harmful algae can make swimming in lakes, rivers and streams dangerous. And we often see the threat rise in the summer.

Why the summer? Well, the same weather that pushes you toward the water during this notoriously sweaty season also amplifies the risk factor.

Weather's role

There are three key elements to summer weather that play a part:

1. Pop-up thunderstorms: Heavy rain washes pesticides, sewage and animal waste into lakes and rivers. Increasing the amount of these contaminants makes the water more dangerous for you.
2. Heat: The hot summertime temperatures warm up the water, making the perfect breeding environment for bacteria and algae. They can multiply quickly to dangerous concentrations.
3. Sunshine: Remember photosynthesis from biology class? Algae thrives and multiplies when it's sunny too.

How to stay safe

• Go to the EPA website "How's My Waterway?" to check if the lake or river you are going to is under advisory or closed for health or safety reasons
• Avoid any water that is cloudier than usual, discolored or has a bad smell as well as any water with significant, algae growth on the surface
• Don't swim in areas near drainage pipes
• Cover any open wounds with waterproof bandages
• Don't swallow any of the water
• Stay home if you are already not feeling well

Caitlin Kaiser graduated from the Georgia Institute of Technology with both an undergraduate and graduate degree in Earth and Atmospheric Sciences before starting her career as a digital meteorologist with weather.com.