For most of its length it flows through a wide valley with low banks, formed by the glacial activity that shaped both rivers' watersheds.
(it; flows; through a wide valley with low banks) (low banks; formed; by the glacial activity) (the glacial activity; shaped; both rivers' watersheds)
it low banks the glacial activity
flows formed shaped
through a wide valley with low banks
by the glacial activity
both rivers' watersheds