Complex sentence: In 1992, Roosevelt spent his 50th birthday rafting down the treacherous 1,000-mile Rio Roosevelt in Brazil-a river previously explored by his great grandfather during 1914 in the Roosevelt-Rondon Scientific Expedition.
Split into simpler sentences: In 1992, Roosevelt spent his 50th birthday rafted down the treacherous 1,000-mile Rio Roosevelt in Brazil. This was the river that his great-great grandfather explored in 1914 in the in the Roosevelt-Rondon Scientific Expedition.