One very successful guy on this site used bags of sakrete cement. You must dig back into the bank and start in the void or it will wash out. Stack them about 4 deep and 4 wide. Or, I guess you could do more. I do know that the higher you get, the more likely you are to lose it.

You need a clay bottom or good sandy loam. If you try it on a sandy or rocky or gravel bottom, it will run under it.