water wave question...

Ripples travel forward due to vibrations of water molecules. These vibrational motion will be transmitted vertically downward to water molecules underneath the surface.
If the depth of water is shallow, there is strong interaction of the bottom with the vibration of water molecules. This interaction will slow down the propagation of the ripples.
In deeper water, water molecule interaction with the bottom is less, and the ripples can travel faster.
Notice that the shortening of wavelength when ripples travel from deep to shallow of water is due to the slowing down of a wave crest when it reaches shallow water, hence the crest behind (which is still in deep water and has a faster speed) will catch up with the crest in front. This resulted in a shortening of distance between the two crests, i.e. shortening of wavelength.
Therefore, shortening of wavelength when ripples travel in shallow water is a CONSEQUENCE of the slower speed of ripples, not the CAUSE of it.