Mikvaos 1:7

Of higher degree than the previous types of mikvahs is a mikvah containing 40 seah (about 140 gallons), in which people may immerse themselves and other objects. Higher still is a spring whose own volume is small but which has been increased by a greater volume of drawn water. This is equal to a mikvah in that it purifies through standing water, and it’s equal to a spring in that one may immerse in it regardless of its volume.

Mikvaos 1:8

Higher still is “smitten” water, which can purify even when flowing. Even higher is “living” water because this can be used for the immersion of a zav, the sprinkling of a metzora and preparing the water with the ashes of the parah adumah (red heifer).