Beryllium (element)
鈹 is a rare kanji that refers specifically to the chemical element beryllium. It has no other common meanings.
鈹 combines the metal radical 金 with the phonetic component 皮. It was created as a character for the element beryllium, likely chosen for its sound rather than any semantic link to skin or leather.
The metal radical 金 hints at a metallic element. The right side 皮 (skin) is just along for the sound, but you can imagine a lightweight metal so rare it's like a thin skin of metal.
For カワ, think of a river (川, かわ) flowing with beryllium: 川 -> カワ, and the metal 金 is found in the riverbed.