She hadn't looked old enough to cross the street by herself.
She still didn't look old enough to cross the street by herself.
They won't try again to cross the river by themselves.
Why did she cross the park by herself in the dark, for example?
Not one of them can cross a river by itself.
Nobody would cross the river by either of those ways for a good long while.
The boat cannot cross the river by itself with no people on board.
In summer it was usually possible to cross the river by foot.
The army was ready to cross by about 12 August.
Why would she want to be crossing water by boat?