Here's my experience with EWM.

Early summer 1/2 the pond covered in EWM, removed it manually weekly, dozens of tractor bucket loads, then as esshup said would be the case - by the end of summer the whole pond was covered.

The treatment plan,
A heavy dose of pond dye over winter, at ice-out following spring when growth was low, the Sonar application! EWM eradication lasted 3 years, some spot treatment just starting now.

One note,
Good aeration and adding beneficial bacteria since then has removed over a foot of muck (fertile ground for weeds), pond now has mostly a hard sand bottom now. The little bit of EWM seen recently doesn't look as hearty, has been easily knocked out with spot treatments.