I go with more of an anchor/lanyard type system.
First, I lash a bunch of trees together. Then, I figure our how much depth of water the clump will be place in. Then I cut a rope a few feet longer than the max depth of the water. I tie the clump of trees to a cinder block with that long section of rope.
I my case, the clump will move a bit, but never more than a few feet in either direction until it eventually sinks.
