13 Why should animals become adapted to live in fast-flowing
water when there are regions
of slow-moving water in a
river?

If the animals adapted to fast flowing are found in slow moving water- it could be: 1) They are temporarily dislodged & will get back to its original position or search for a suitable place. 2) If the niche is the same, slow moving water might be only a temporary phenomenon.

explanation:

the fossil record extends back to a dog-like ancestor some 55 million years ago, which gave rise to the first horse-like species 55 to 42 million years ago in the genus eohippus.