Complex sentence: The Whitmans reached the Walla Walla river on September 1,1836 and found a mission to the Cayuse Indians at Waiilatpu in the Walla Walla Valley.
Split into simpler sentences: The Whitmans reached the Walla Walla river on September 1,1836. They found a mission to the Cayuse Indians at Waiilatpu in the Walla Walla Valley.