Refers to the act of crossing a river, often on foot or by wading. Can also imply a view or location across a river.
Mount Fuji can be seen across the river.
川越しの道はぬかるんでいた。
The path across the river was muddy.
Compound of 川 (kawa, 'river') + 越し (goshi, 'crossing'). The reading かわごし is a straightforward combination.