Q. One can't float across a river with Mei Chatas but he can walk across. That is, as long as his feet are touching the ground he is permitted to carry Chatas Water or Chatas Ashes across the river. What is the difference between floating and walking?
My Best,
Mark May, Raanana
Really, according to Torah law, there is no difference, and mid'Oraisa one may take the Chatas water or the Chatas ashes over the river even in a boat and not merely if one's feet touch the water.
However, there is a Rabbinical decree involved here, simply because of the bad experience someone had when he carried Tahor items across the Jordan River on a boat and a k'Zayis of a corpse was found on the floor of the boat. From then on, Chazal imposed a decree that one may not transport Chatas water and Chatas ashes across the river.
Since this is only a Rabbinical prohibiton, they only made their decree apply in the exact same way in whch the original "accident" happened - namely, only if one travels on a boat, not if one walks through the river.
Kol Tuv,
Dovid Bloom