At least one of the half-dozen sewage treatment plants nestled along the Merrimack River flushed wastewater into the waterway during the powerful nor’easter that ravaged coastal communities and dumped several inches of rain across the state.Haverhill sewers overflowed in two locations Friday “due to rainfall and high flow,” one emptying the excess wastewater into the Merrimack and another into Little River, which flows into the Merrimack.