The Rambam holds the Atlantic ocean and all rivers can not be used for the Red Heifer. [Beginning of Laws of the Red Heifer.] [note 1] The Raavad asks why does the Rambam abandon the Mishna and go to the Tosephta? After all the Mishna only invalidates  four rivers in Israel.(The Tosephta invalidates all rivers.)

Rav Shach explains the Rambam is going with the idea of the Gemara that all rivers receive from the Atlantic. 

This all for me was a bit of a surprise because I thought all rivers are fed from springs. Well that is apparently the argument here.

That means the Tosephta is holding that all rivers have a category of a "collection of rain water" and not a spring.

Bu this still leaves me wondering why the Mishna only invalidates four rivers in Israel. Obviously the Mishna holds that all rivers [besides those four] in fact have a category of a spring. The point of the Raavvad still looks valid.

[note 1] The ashes of the Red Heifer are put onto "living waters" in order to be sprinkled on someone who has touched a dead person. That is a requirement before that person can come into the Temple or eat of the sacrifices.