Beach flags indicate whether the water is safe
A yellow flag, like a yellow traffic light, is there to alert anyone in the area to be cautious. If you see a yellow flag on a beach, it means to be more cautious than usual when in the water, even if you are an expert swimmer. If you don't swim well, you may want to skip swimming or entering the water while the caution flag is flown.
The meaning of the yellow flag
A solid yellow flag posted at a beach means "medium hazard" due to water conditions. It could mean rough waters and large, powerful waves caused by high winds or storms in the area, deeming the area unsafe for weak or even moderate swimmers. Yellow may also indicate strong currents or an undertow that could prove dangerous to those swimming in the area.
The yellow flag serves as a warning for all water activities – not just swimming – such as surfing, windsurfing, kayaking or paddleboarding if such activities are allowed nearby. Those with little or no experience with either swimming or using such watercraft should wait until conditions become more safe.
Other common beach flags
Many lake and ocean beach areas use the same three solid-colored flags, including the yellow flag, for the most common beach conditions. A green flag stands for "low hazard" or safe conditions, but beachgoers should still proceed with caution. A red flag means "high hazard," which could also mean strong currents and tall waves. This flag also means "no swimming" at many beaches. Some beaches use two red flags, one flown over the other, meaning that everyone should absolutely stay out of the water.
Other beach flags featuring yellow
Some beaches use a more elaborate flag system incorporating more than three flags, depending on conditions. A yellow flag with a black circle in the center means any non-powered watercrafts are prohibited in the area. This means surfboards, stand-up paddleboards, kayaks and the like cannot be used.
A flag with red on top and yellow underneath indicates the area is a designated swimming zone with a lifeguard on duty. In some cases, the beach team will post two such flags; one at each side of the patrolled swimming area.
Purple flags for ocean beaches
Some ocean beaches also use a purple flag that isn't generally flown on freshwater lakes. The purple flag means that dangerous marine life has been spotted in the water, such as stingrays or jellyfish. This flag is not used to indicate sharks in the area. If sharks have been spotted nearby, the beach team will instead fly a red flag or double red flags.