The Great Salt Lake, the largest salt lake in the Western Hemisphere, is a remnant of ancient Lake Bonneville. As a terminal lake with no outlet, minerals accumulate as water evaporates, resulting in famously high salinity. This high salt concentration is the direct reason why the lake rarely freezes, even during cold Utah winters. Ice formation is possible, but it depends entirely on localized conditions and the lake’s ever-changing chemistry, particularly the dramatic difference in salinity between the northern and southern sections.
The Mechanism of Freezing Point Depression
The reason salt water requires a lower temperature to freeze than fresh water is known as freezing point depression. Pure water freezes at 32°F (0°C) when its molecules align into a rigid, crystalline lattice structure. When salt is dissolved in water, it dissociates into charged ions that interfere with the water molecules’ ability to organize into the necessary ice crystal structure. This means the water must be cooled to a temperature far below the normal freezing point to overcome this interference.
The depression of the freezing point is directly proportional to the concentration of dissolved particles. Since the Great Salt Lake contains significantly more dissolved minerals than ocean water, its freezing point is much lower than the approximately 28.4°F (–2°C) of average seawater.
Salinity Differences Between the North and South Arms
The Great Salt Lake is physically divided by the Lucin Cutoff, a railroad causeway constructed in the late 1950s. This structure restricts water flow, creating two distinct bodies of water with dramatically different chemistries.
The South Arm, known as Gilbert Bay, receives almost all the fresh water from the Bear, Weber, and Jordan rivers. This continuous inflow dilutes the salt content, maintaining a salinity level much lower than the North Arm. The South Arm is the section where ice formation typically occurs, though the exact concentration fluctuates based on lake level and tributary inflow.
The North Arm, or Gunnison Bay, receives almost no direct freshwater input, making it a hypersaline environment. Salinity here is often near saturation, historically averaging around 317 grams of salt per liter, nearly ten times saltier than the ocean. Because of this extreme salt concentration, the North Arm’s freezing point is so low that it virtually never freezes over.
Historical Instances of Ice Formation
Freezing events are rare, requiring prolonged, extreme cold temperatures combined with significant salt dilution. The most extensive ice formation occurs when the lake has experienced high water levels, which naturally lowers the overall salinity of the South Arm. This allows the water’s freezing point to approach a temperature reachable during a sustained winter cold snap.
For example, during the early 1980s, an unprecedented rise in water level lowered the salinity enough that ice formed along the shoreline and in the shallower areas of the South Arm. Ice is often observed when fresh water from tributary rivers freezes on the surface before fully mixing with the dense brine below. This process results in floating ice sheets that are sometimes pushed ashore, a phenomenon locally known as an ice shove.
The less saline South Arm requires a prolonged period of sub-freezing air temperatures to dissipate the heat stored in the large volume of water. The formation of a stable, widespread layer of ice remains a rare event, directly tied to the lake’s fluctuating water levels and resulting salinity concentration.