Below is the direct translation from a 3rd grade reading level children's book on kappa. I have not yet compiled the information, so much of it is repetitive; the main value at this point is that it gives an overall picture of the modern idea of what kappa are, and how they act. As I dig more, I suspect that the ancient idea of what a kappa should be will differ; it's already clear that many of the "kappa" stories I've been translating are not kappa stories... just stories that included some sort of water monster that was later re-named a kappa. It'll take some time to sort this all out. In the meantime, here's what 3rd graders are told about kappa...
Rivers & Ponds
Kappa have an undeniable relation to water. Nearly all kappa live in muddy areas of rivers and ponds, and yet, like frogs and such, are just as comfortable out oif the water on dry ground.
They are weakened by the cold of Winter, so Summer is when they become active.
It is also said by some that they are assistants of the heavenly Dragon King, who brings all rain and water to the Earth.
In the region of the island Kyushuu, stories about kappa living in the ocean often appear.
In Kumamoto Prefecture, there is a monument that commemorates the arrival of kappa that came all the way from the oceans near China. In Fukuoka Prefecture, there is a shrine dedicated to a female kappa who was once the protector of the local fisher women who dove for shellfish. This kappa's name was "Oo-oyabun Amagozen", which can be roughly translated as "Big Boss Ocean Front" [which is as strange a name in Japanese as it is in English].
It is said there are still many stories about kappa active in the oceans.
Kappa live in ponds and rivers during the Summer, but in Fall and Winter there are also kappa that live in the mountains, or so it is said.
Once a kappa starts living in the mountains, they are refered to as Yamawaro [rather than kappa].
Because they live in water, it's only natural that kappa are outstanding swimmers. At swimming, kappas have no equal. Thanks to their webbed hands and feet, they are not only skilled swimmers, but very fast swimmers as well.
It is well-known that kappas often pull horse and humans into rivers.
Despite having bodies small as children, they have great physical strength. Because kappa are proud of this strength, Sumo is one of their favorite pasttimes. The secret of this great strength is the water in the bowls on their heads. This is the source of all their power.
Transforming into Humans
Kappa sometimes transform into humans. What kind of humans? Here are the "characters" they can transform into.
A young apprentice priest, a child, a young woman, a youth proud of his strength, or a beautiful lady. In all cases they will love to join into a Sumo match (though this is especially true for those disguised as children).
Transforming into Objects
It is said that when a pretty ball comes floating down a river following the flow, it is dangerous to try to retrieve it, for it might be a kappa in a transformed state. Putting out a hand carelessly could result in being pulled into, and under, the water. When any pretty object comes floating down the river, be careful!
They Make Effective Medicines
There are people who have learned how to make an effective medicine, good for cuts and bruises, from thankful (though mischievious) kappa that they had caught and then released. Even now, this medicine is for sale. There are also people alive who have learned the techniques for skillful bonesetting from kappa.
They Have Treasure
At the bottom of their rivers, kappa hide many small objects and gold.
It has been handed down in many tales that people who were about to be pulled into the rivers by kappa tricked the creatures instead, and gathered a lot of their gold.
They Like Cucumbers
Cucumber rolls are called "kappa" rolls because kappas love cucumbers. At the start of Summer, the first cucumbers of the season are let go in nearby rivers to keep the kappa pleasant and happy while the first swimmers of the season enter the water. This ritual gives mid-summer swimmers peace of mind.
They Like to Play Tricks
It is said that two pranks that kappa often play is pulling horses that are eating grass next to rivers into the water, and patting people's buts when they are not looking. This last one is a bit perverted, isn't it?
They Like Sumo
Kappa like Sumo, and they are very strong too. First off, kappa are never beaten by humans in a straight Sumo match... but, kappa have a weakness. At the start of the match, when opponents ritually bow to each other, the water in the saucer on the kappa's head will spill out; in this way, the kappa can be forced into giving up.