Walter Jackson, age 11, of Flat Rock, North Carolina, for his question:
What caused Great Salt Lake?
The life story of any lake is a theme of precipitation and evaporation. The clouds add water and streams drain into it the run off from surrounding slopes. The sun and the dry air evaporate tons of moisture from the lake's surface. This same theme prevailed through the life of Great Salt Lake. However, as in all lakes, the earth's climate and geography molded its features. And what they did to this remarkable lake was something special.
The groundwork was laid when the massive Rockies were rising, perhaps many millions of years ago. This region of Utah was in a fault zone and successive earthquakes caused a huge slab of the earth's crust to sink down and form a large basin. We can assume that streams drained down into the hollow and there is evidence that tributaries from a mountain canyon carried in deposits from ice age glaciers.
The evidence of more recent events is more clearly etched in the territory around the present Great Salt Lake. Around its salty shores there are old discarded beaches, massive deltas and wave washed cliffs, now high and dry and often far up the slopes. One deserted old beach is now 1,000 feet above the present level of the lake. Obviously a far larger lake once filled the hollow basin and toward the northern end, a deserted canyon suggests that its waters drained into the Snake River and flowed with it to join the sea.
In 1833, the features of this ancient lake were surveyed by a United States Army captain. In his honor it was named Lake Bonneville, a name still used when we're referring to the huge prehistoric lake.
The present lake occupies about 940 square miles in northwestern Utah. But a longtime ago it stretched hundreds of miles farther south and spilled over the border into what is now Nevada: To the north, it reached arms into what is now Idaho and found the channel that led its waters to join the Snake River.
We can assume that its waters were fresh because they merely rested a while in the great hollow, on their way from the slopes to the sea. Because the theme of lake survival is precipitation and evaporation, we can make a guess about when old Lake Bonneville reached its prime. Cool air would slow down its surface evaporation, plenty of runoff would fill its huge basin. A suitably cool climate existed toward the end of the last ice age, when countless streams gushed from melting glaciers.
During the post glacier period, drainage was reduced and the lake surface was exposed to thirsty desert air. The lake shrank and shrank below the level of the channel that formerly drained it to the Snake River. Streams still emptied into the dwindling lake, but they did not contribute enough water to equal the moisture lost by evaporation. Chemicals dissolved in the water no longer were carried out to sea. They stayed in the lake and its salty waters got saltier.