RobH_en_tn/psa/024/002.md

For he has founded it upon the seas and established it on the rivers

Here "founded it upon the seas" and "established it on the rivers" mean basically the same thing. The Hebrews of that time believed their land was supported by the oceans and deep underground rivers. Alternate translation: For he formed its foundation on the seas and built it on the deep waters" (See: rc://en/ta/man/translate/figs-parallelism and rc://en/ta/man/translate/figs-explicit)

the seas ... the rivers

These phrases are used together to refer to the huge deep ocean below the earth.

on the rivers

"the water that is deep below"