Find out how big the watershed is? With a normal watershed you can apply the 2 gallons of Sonar now and then follow up next spring with a gallon spread out through a couple treatments to keep the stuff from reforming. It takes a much lower dose of fluridone to keep stuff from growing and mating than it does to kill mature plants. I would still recommend killing the mature plants yet this summer to set the stage for next year. Watermeal is the biggest concern that needs to be the focus of eliminating. Once that is eliminated (2-3 years) you can scale back the fluridone and get rolling with good native vegetation.
Also would suggest applying some cheap bacteria packets 21 days after your Sonar treatment to help eat up the decaying vegetation. The other stuff will start dieing much quicker than the watermeal. 12 packets of bacteria per acre, about $3 per packet.
