To prevent the mainline from going sideways when you tension it, use two screw anchors with eyelets on top. 30 or 36 inch length should be plenty.
Screw one to the river bottom on the upriver end to be a permanent fixed point for your mainline. Then use what ever weights you choose to tie the downriver end of the mainline to. Nothing tricky about the weight except you want something much denser than water, like steel or concrete. 50 pounds or so.
(Remember, for every gallon of water volume displaced by your weight, your weight loses 8.34 pounds when underwater).
Once you have the mainline tensioned, install the 2nd screw anchor directly under the mainline anchor rope, about 15-20 feet upriver from your anchor weight. Then, pull the mainline anchor rope down and into the eyelet.
Now, if the river drops, and you need to pull a lil tension, you won't change the course path at all. It will remain centered between your turn buoys.
You just need a couple scuba divers and a short piece of pipe to screw the anchors in the river bottom. If the river bottom is deep with soft silt you might need longer screw anchors to get a bite in solid soil. These anchors are not very expensive.
You also might want to attach small sub buoys on the 2 screw anchors and the down river weight in case you ever need to find them again.