i lost a mushroom anchor on the left channel of miller island a couple years ago trying to stop my canoe in shallow water (1 gen). it was the anchor or us, my canoe nearly tipped over taking my brother, myself and gear with it. this was not the first anchor we had lost so i decided if was going to keep buying anchors i might as well just dump my wallet out into the river. now i use two window weights bolted together at the top with a copper bolt, and a hose clamp in the middle to keep them together. i put a piece of a thick plastic straw over the bolt so the threads don't chew the rope, and painted them to help stave off rust. they have never hung up so bad i can't retrieve them, but they will not stop you very well in moving water. they also will stand up and lay back down, but i like the design as it almost never gets hung. i have not found the perfect answer to this problem yet. if i must stop in moving water i usually tie off to a tree along the bank for a bit
i used to use an old, heavily painted brake rotor sometimes for stopping mid stream but have learned its just not safe to do so in a canoe, especially with other boats around putting off a wake. also no matter how much i painted it i could not get it to cover the inside well enough to stop it from leaching rust into the river. i still carry it as a backup but am going to replace it with something better soon.