This river forms the boundary between the two states.
The kanji and the word are exactly the same. That means they share meanings as well.
Since this word is made up of a single kanji, it probably uses the kun'yomi reading. You did not learn this reading, so here's a mnemonic to help you to remember: You walk up to the boundary. You're there to pick something up to take it across. This thing you're smuggling across the boundary is a sockeye (さかい) salmon. You take it across the boundary and sell it for a big profit.