Many years ago, I worked as a tour guide in Alaska, falling deeply in love with the state. One of my favorite drives was along Turnagain Arm, a long and shallow branch of Cook Inlet, a beautiful landscape that is home to a fascinating natural phenomenon. Bore tides occur when an incoming high tide collides with the outgoing tide in a narrow channel, generating a turbulent wave front. Getty Images photographer Streeter Lecka was recently lucky enough to spend six days on Turnagain Arm, photographing the brave souls who venture out onto the mudflats to ride these waves. Waves can reach as high as 10 feet tall, crashing over calmer waters, moving upstream at 10-15 mph. Gathered here are some of Lecka’s images of the surfers riding the bore tides of Turnagain Arm.