New Defender's Study Bible Notes
7:20 mountains. The words “high hills” and “mountains” are the same in the original Hebrew. The waters were 15 cubits (22.5 feet) above the highest mountains, patently including Mount Ararat, which is now 17,000 feet high. In the “local-flood” theory, Mt. Ararat would have had the same elevation before and after the Flood, but it should be obvious that a 17,000-foot flood is not a local flood!
7:21 moved upon the earth. “All flesh” died that moved on land. In a local flood, at least most of the animals (certainly all the birds!) would escape to higher ground.