I was part of a US Army Corps ...

I was part of a US Army Corps of Engineers research project in the mid-1970's that assessed the use of two herbicides to control water weed (Elodea and Egeria) and several species of pondweed Potamogeton).  We found that with the right herbicide application matched with the target species, with control, you can manage infestations effectively with no residual herbicides in potable water supply reservoirs.  Long term success depended on targeting both the vasculars and the algae bloom that would occur from ortho-PO4's released into the water column after chlorosis occurred post vascular weed control.  Since then I have had good results on various weed control (other species) using biologics (including aquatic insects), mechanical control, water level control and such.  In short, a number of effective weed control options exist, and effectiveness varies as a function of weed, water body geophysics, water chemistry, nutrient control within the watershed, etc.   Good luck!