what could be the reason for the salinity of this water body?
Most of the salts dissolved in the lake and deposited in the desert flats, lake's waters are slightly enriched in potassium and depleted in calcium.
Great Salt Lake
Most of the salts dissolved in the lake and deposited in the desert flats around it reflect the concentration of solutes by evaporation; Lake Bonneville itself was fresh enough to support populations of fish. More salt is added yearly via rivers and streams, though the amount is much less than the relict salt from Bonneville.

The salinity of the lake's main basin, Gilbert Bay, is highly variable and depends on the lake's level; it ranges from 5 to 27% (50 to 270 parts per thousand). For comparison, the average salinity of the world ocean is 3.5% (35 parts per thousand) and 33.7% in the Dead Sea. The ionic composition is similar to seawater, much more so than the Dead Sea's water; compared to the ocean, Great Salt Lake's waters are slightly enriched in potassium and depleted in calcium. Dissolved ions do not necessarily increase or decrease in step with changes of total dissolved solids. For example, in October 1903 dissolved solids tallied 27.72% and by February 1910 they were down to
what kind of fish are in the great salt lake in utah UNANSWERABLE is it a freshwater lake or a saltwater lake? Salt water lake
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Great Salt Lake [SEP] Salinity
Most of the salts dissolved in the lake and deposited in the desert flats around it reflect the concentration of solutes by evaporation; Lake Bonneville itself was fresh enough to support populations of fish. More salt is added yearly via rivers and streams, though the amount is much less than the relict salt from Bonneville. The salinity of the lake's main basin, Gilbert Bay, is highly variable and depends on the lake's level; it ranges from 5 to 27% (50 to 270 parts per thousand). For comparison, the average salinity of the world ocean is 3.5% (35 parts per thousand) and 33.7% in the Dead Sea.