河 is a phono-semantic compound: the water radical gives the meaning and 可 gives the sound.
Tips
memory
形 (form) is the meaning part and 声 (sound) the sound part. In 妈, 女 says "woman" and 马 gives the sound. Roughly four characters in five are built this way, so the sound part is a useful clue to the reading.
usage
The two halves are the 形旁 and the 声旁. Compare 会意字, where every part carries meaning and none gives the sound.
