Complex sentence: He is also considered one of the first men of European descent, along with Étienne Provost, to see the Great Salt Lake, which because of its salinity, he first believed was an arm of the Pacific Ocean.
Split into simpler sentences: He is also considered one of the first men of European descent, along with Étienne Provost, to see the Great Salt Lake. Because of its salinity, he first believed it to be an arm of the Pacific Ocean.