I saw a crab walking home from work the other night. We're by a river, but I was probably 500 feet away from the river when I saw this one. I apologize for the image quality, all I had was my terrible Ipad camera and no flash. I have provided the original image and one with brightness and contrast modified to make the crab easier to see. It's about 2.1 inches from leg to leg, I threw a 500 yen coin by the crab as a reference, but cropped it from the image.
I am located directly across from Haneda airport, which is on Tokyo bay. The river is close enough to the ocean that we get tides. I'd say the river level varies by as much as a meter, and there are many many crabs on the exposed riverbed at low tide. But again, this crab was far away, and would have needed to climb a three foot tall vertical wall at the river's edge.