[RWebb]

yes, "a property owner who alters the natural watercourse is generally liable to downstream owners"

the problem is proving it & what it was like beforehand

you are lucky to be in a city, that will make it a lot easier

if that does not work, then you will need to find an attorney who specializes in water law in your area

it is a very insular & specialized area - in the West, cases can drag on for many decades

Good Luck!