Illegal garbage dumping? Again? We need to know some law about this. All runoff goes straight into the creek?

We & our neighbors live on hills across from each other. Down in the bottom runs a creek & the beginning of our driveway. Their driveway is on their hill. Our property line was about a foot this side of their carport 20 yrs ago. Now they have yard there & can even drive around this carport. They have filled up the creek bank with tires & garbage. We asked then argued & not until I began to video tape them & the other people they let dump shingles, etc., over all these years they finally stopped. The property first belonged to my dad, now it's mine. We cannot afford to have it surveyed. We're on a limited fixed income. We even went as far as to tell them that we didn't want to cause any trouble & that if they stopped, until it had to be surveyed or something, they could leave the big space beside the carport. So, the problem is, they have started dumping again. It's not alot & right now it's on their property, throwing it over the creek bank, we have to look at it. Thanks, Becky