There's a bridge under that water and debris. Heavy rains resulted in rivers overflowing both bridges on the 9 km dirt road from the main highway to the Arenal Observatory Lodge. Many cars and minibuses were unable to cross, but fortunately our guide was driving a Land Rover, which made it easily. See the pothole photo above for another view of this road. (There's also a short video linked from that picture showing what it felt like to drive on this road.)