Interactive coastal cross-section: a canoe moves toward the beach over a sandbar or through a deeper channel, showing waves slowing and their crests crowding together as the water gets shallower.

Slow, crowded waves mean the bottom is close

Out at sea, swell runs long and fast and takes no notice of the bottom. Once the water is shallower than about half a wavelength, the sea floor drags on each wave: it slows, its crests crowd together and it steepens. Coastal Aboriginal Peoples have read these signs for generations to judge depth and find reefs, sandbars and channels. Paddle the canoe toward the beach and watch the crests crowd together.

Coast cross-sectiondeep water
Move the canoe and watch the wavelength change. Same swell, same period, all the way in.
Depth here 28 m
Canoe distance from the beach200 m
Open seaMid wayNear the beach
Wave speed here
0 m/s
Wavelength here
0 m

Know This

In shallow water a wave's speed is v = √(gd), and with its frequency fixed, a slower speed and a shorter wavelength both mean shallower water.