Rashi on Genesis 14:3/רש"י על בראשית י״ד:ג׳

1עמק השדים VALE OF SIDDIM — So it was named because it contained many fields (שדות)

2There are many Midrashic explanations of this name. הוא ים המלח THE SAME IS THE SALT SEA — After some time the sea flowed into it (the vale) and thus the Salt Sea came into existence. The Midrashic explanation states that the rocks that surrounded it had cracked and streams thus flowed into it (Genesis Rabbah 42:5).