Welcome to PB ! Assume all is frozen now given the location. I would suggest as a first step do a survey yourself by small boat taking depth readings. If ice is safe and you have an auger you can do it now. You can use a weight and marked string. Take pics of the dam and vegetation. Looks like vegetation removal/reduction and muck removal is a needed step. What general approach to take is dependent on if there are fish and if you want to save them. Sometimes raising the water level is easier than muck removal.