A major rescue operation involving a coastguard helicopter and a lifeboat was launched on the River Conon between Dingwall and Conon Bridge.
The woman was initially spotted near Ferry Point, where there is a coastal footpath.
She was plucked from the water by the Inverness-based coastguard rescue helicopter and airlifted to the city’s Raigmore Hospital.
But the pensioner – who has not yet been named – died later.
Police said there were not thought to be any suspicious circumstances and that her next of kin had been contacted.
The tragedy happened at the end of a week in which an Aberdeen mother and son died when a family day out at the beach turned to horror and several other people lost their lives in separate accidents around the UK’s coasts.
RNLI volunteer crewman Michael MacDonald said: “The water is at the mouth of the Conon. The tide was on its way out.
“We got there as quickly as we could to assist in any way we could. The helicopter from Inverness took the woman straight to Raigmore.
“Our thoughts do go out to the family at this sad time. It could have been a tragic accident, unfortunately. How the lady ended up in the water I’m not sure.”
The RNLI is currently promoting a Respect the Water campaign.
Mr MacDonald said: “It’s a major campaign for us, especially with all the deaths over the past seven to 10 days around the coast.
“It doesn’t have to be deep water. It doesn’t have to be fast flowing. Water itself can be dangerous. Just treat it with respect.”
Local councillor Margaret Paterson said the community would be devastated by the latest tragedy, which happened around 6.20pm on Saturday, in a black 10 days.
Separate road accidents in the area claimed the lives of former NHS Highland chairwoman Caroline Thomson and local garage owner Noel Gilbert.
Councillor Paterson said: “We’ve had so many tragedies this last while. This is another tragedy for the community and everyone is desperately sorry to hear this.
“Another family’s going to be heartbroken and my thoughts and prayers go out to family and friends of this lady.
“Dingwall and the surrounding area have experienced a number of deaths in the past week and it effects everybody. Everybody is distressed to hear it.”
Meanwhile, Oban’s RNLI lifeboat was launched to take a man who fell and injured himself at Glensanda quarry to hospital.
The super quarry on the Morvern peninsula is only accessible by boat.
Paramedics accompanied the lifeboat crew and the man was transferred to a waiting ambulance when they arrived back in Oban early on Saturday.