Here, I feel like Ellie is describing what it's like to start a new relationship. You feel "stripped" and exposed to be so intimate with this new person. "The river" could simply be her way of describing a relationship: something fast-paced that can consume you (whether that's good or bad).
Less generally, Ellie could also be talking about what it's like to start a relationship while being in the public eye. She and her boyfriend, feeling "stripped" and exposed, fall into "the river" that is the media.
"Cover your eyes
So you don't know the secret
I've been trying to hide"
Here, Ellie could be addressing her new boyfriend. Like in every relationship, there are certain secrets you don't want your partner to know.
Again, Ellie could be addressing the media and the "secret" could simply be her love-life that she would like to keep private rather than public.
"We held our breath
To see our names are written
On the wreck of '86"
Here ellie is expecting the worst. She thinks her relationship will inevitably end and she’ll be grouped together with the other twenty-somethings of her ‘wreck’ of a generation.
"That was the year
I knew the panic was over"
I actually don’t think she’s referring to 1986 here. Instead, I think she’s talking about the year she spent with this person. She no longer has to worry about being compared to her ‘wreck’ of a generation. She feels she’s found the one: panic over.
"Yes since we found out
Since we found out
That anything could happen"
This is where the relationship, in my opinion, goes wrong. "Since" coming to terms with the idea that this person may be the one, their seemingly perfect relationship comes to end, at which point they realise that "anything could happen"
"After the war we said we'd fight together"
After what seemed like a 'war' of a break-up, they both agree to remain friends.
"I guess we thought that's just what humans do"
This could mean that they both agreed that it was the most kind and 'human[e]' thing to do
Or maybe it's a bit more cynical, implying that people always end up forgiving those who hurt them.
"Letting darkness grow"
Here Ellie suggests that remaining friends with her ex was not easy. The 'grow[ing] darkness' implies perhaps that it caused more pain and sadness, at first, than happiness.
"As if we need its palette and we need its colour"
Although this was a hard time for her, she felt like she still 'need[ed]' this person in her life, even though he made her feel 'dark.' And eventually she manages to find 'colour' in this 'darkness' - suggesting things are gradually on the mend between them.
"But now I've seen it through
And now I know the truth
That anything could happen"
Pretty self-explanitory, she saw it through it through and now she knows that 'anything could happen' - in this case, being friends with someone she used to love and with whom she had a difficult break-up.
"Baby, I'll give you everything you need
But I don't think I need you"
This could be what's left of their relationship. They're friends. She's there for him if he needs anything. But just as a friend. She doesn't need him anymore, romantically.
"I know it's gonna be
I know it's gonna be"
Ellie repeats this line many times, and to me, it seems like Ellie is at a loss to describe what their relationship/friendship is going to be. She doesn't 'know it's gonna be' good, she doesn't 'know it's gonna be' bad. All she knows is that 'it's gonna be' i.e. that whatever they have is going to continue - reinforcing the theme that 'anything could happen'.
"Stripped to the waist We fall into the river"
Less generally, Ellie could also be talking about what it's like to start a relationship while being in the public eye. She and her boyfriend, feeling "stripped" and exposed, fall into "the river" that is the media.
"Cover your eyes So you don't know the secret I've been trying to hide"
"We held our breath To see our names are written On the wreck of '86"
"That was the year I knew the panic was over"
"Yes since we found out Since we found out That anything could happen"
"After the war we said we'd fight together"
"I guess we thought that's just what humans do"
"Letting darkness grow"
"As if we need its palette and we need its colour"
"But now I've seen it through And now I know the truth That anything could happen"
"Baby, I'll give you everything you need But I don't think I need you"
"I know it's gonna be I know it's gonna be"
This x5
This x5