They had just traveled over the Rockies. I'm sure they got over the mountains and saw this giant lake and famously pronounced "This is the place". But when they got to the lake, what happened? Just a thought as I was flying in today.
Fremont explored the areas around the Great Salt Lake four years before the Mormons arrived. Mormon leadership at least knew what was there and that fresh water was available from streams entering the lake.
The famous pronouncement above is a common misquote. Brigham Young is thought to have said:
"It is enough. This is the right place. Drive on."
This has been simplified over time to the now oft-repeated "This is the place."