Lynn Arave of the Deseret News recently wrote a short article about marathon swimming in the Great Salt Lake. A link to the story was also posted on 10K Swimmer.
According to the article, the first marathon race in the Great Salt Lake was held in the summer of 1919. There were intermittent races throughout the 20's and 30's. The last time a race was held at the lake was in 1940. Because of low water levels and the start of World War II, the races were discontinued.
I am planning on doing a series of swims in the Great Salt Lake this summer (anyone who cares to join me is more than welcome) and would love to get as much information as I can about the history of swimming in the lake (or anywhere in Utah for that matter).