The salinity of Great Salt Lake is determined by the amount of inflow (and its salt content) and the amount of evaporation. When there is a lot of inflow, the lake elevation increases and the salinity of the water decreases. When there is less inflow or the evaporation rate is high, the lake elevation declines and the water becomes saltier. In 1959, a solid-fill railroad causeway was constructed across the middle of the lake. The causeway divides the lake into two parts: the north part (Gunnison Bay), which receives little freshwater inflow, and the south part (Gilbert Bay), which receives almost all the inflow. For any given lake elevation, the salinity of Gunnison Bay is always greater than the salinity of Gilbert Bay. The USGS measures salinity periodically at Saltair Boat Harbor and at Promontory (Gilbert Bay) and at Saline (Gunnison Bay).
The USGS also collects salinity data from each of 16 sites in Gilbert Bay at intervals of 2 to 4 weeks. Data collected since August 1995 from USGS Site 3510, located midlake and 6 miles west of Antelope Island and measured at 1-meter depth, are shown in this graph.
Measurements of salinity at the Saline Boat Harbor on Gunnison Bay for water years 1996 to present show a smaller variation than in Gilbert Bay and lack a downward trend. The salinity in Gilbert Bay declined from almost 15% in late 1995 to 11% in late 1997, but salinity in Gunnison Bay fluctuated only 1 to 2% around 28% during the same time.
The high salinity (currently 28%) in Gunnison Bay excludes many of the organisms found in Gilbert Bay. Brine shrimp are often washed into Gunnison Bay and may be able to reproduce in a small area where less saline water enters from Gilbert Bay, but as they are carried by currents into the main body of Gunnison Bay, they die. Brine flies inhabit the shoreline, but the most visible aquatic organisms in Gunnison Bay are the photosynthetic sulfur bacteria that color the water a reddish-purple. Estimates of total biomass of sulfur bacteria in Gunnison Bay range from 180,000 tons to 2 million tons. An alga with red coloration also inhabits the bay, but it is less numerous than the bacteria. The red-purple coloration is easily visible from aircraft using the Salt Lake International Airport.
USGS publications about the salinity of Great Salt Lake