These slimy demons will lure you into the river and suck your blood. You are unlikely to get out alive.
You can recognize a KAPPA from the bowl-like depression on top of its head. The bowl is filled with water and this gives the demon its power.
KAPPAs are, however, extremely polite. So here is what you do. Bow courteously in the approved Japanese fashion. Naturally the creature will have to return the bow. All the water on its head will tip out. The KAPPA is then powerless until the water can be replenished.
That is not all. The KAPPA are crazy about cucumbers. They prefer cucumbers to blood. Bribe them with a cucumber and they will promise you almost anything.