The kappa is an amphibious water-dwelling spirit creature well known in Japanese folklore. He is sometimes regarded as a manifestation of the water-deity suijin and needs to be propitiated with rites and offerings. He prefers still, muddy water but may live in the sea (see Kappa tenno-sai). The kappa is variously described and represented in folk art as a scaly, dark blue creature somewhat like a 3-4 year old boy with a pointed face, webbed and sharp-clawed hands and feet and thick hair. The most significant feature of a kappa however is the saucer-shaped depression on his head which holds water. If the water dries up while the kappa is on land he dies, just as crops die if the supply of water fails.
A Popular Dictionary of Shinto. Brian Bocking.