How did the Great Salt Lake get its salt?

Rains and rivers dissolve salt from salt deposits and transport NaCl in the lake.
The Great Salt Lake has salt because its tributary rivers constantly keep feeding the lake with small amounts of salt. The lake does not have any outlets and water evaporates leaving behind all the salt. Hence the salt has been accumulating over time.
Rains and rivers dissolve salt from salt deposits and transport NaCl in the lake.