Complex sentence: The wagon trails following the south side of the Platte/North Platte River ferried or waded in low water years across the swampy bottomed South Platte River in several places to stay on the south side of the North Platte River where the trails were located.
Split into simpler sentences: The wagon trails followed the south side of the Platte/North Platte River. Wagon trains were ferried or waded in low water years across the swampy-bottomed South Platte River in several places to stay on the south side of the North Platte River where the trails were located.