Horse & Pony Books:
(DODD MEAD [USA] 1968)
ILLUSTRATED BY GEORGE LOH
SUMMARY: Set in a Kentucky river town. 10 year old Sue loves living by the river with her many cats and her orphaned colt Silver Dollar. But the river is not always friendly and when it floods her beloved Silver Dollar is put in danger.
Fairly easy to find in the USA, although prices seem to vary a lot. Can be much harder to get hold of elsewhere.