Hi Glenn, i'll caveat my ...

Hi Glenn, i'll caveat my suggestions - I don't have direct experience with weed management in reservoirs! but I am experienced in how to benefit vegetation growth/reproduction with targeted water management. So some reverse logic:

If you know the different species and their depth/temperature/light requirements this might highlight their weak points that you could exploit with targeted operations of the dam. ie making the water too deep or too shallow or fluctuating in an undesirable manner for the weeds. Seasonality of these operations could be important too as it may prevent the weeds from reproducing. this of course would need to be balanced with what is feasibly possible with your dam!

hope that helps and best of luck with your problem