Complex sentence: The railroad was located on the south bank of the Cascades canal of the Columbia River, from Tanner Creek (near where Bonneville Dam was later built) to the Cascade Locks, which were under construction in the later years of the railroad's operation.
Split into simpler sentences: The railroad was located on the south bank of the Cascades canal of the Columbia River. It ran from Tanner Creek (near where Bonneville Dam was later built) to the Cascade Locks, which were under construction in the later years of the railroad's operation.