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.
This river forms the boundary between the two states.