Complex sentence: Wyeth's party crossed the Kansas River on May 5, founded Fort Hall (July 1834) in southeastern Idaho, and built Fort William on an island in the Columbia River at present-day Portland, Oregon.
Split into simpler sentences: Wyeth's party crossed the Kansas River on May 5, founded Fort Hall (July 1834) in southeastern Idaho. They traveled on to the lower Columbia River, where they built Fort William on an island at present-day Portland, Oregon.