וַיֵּצֵא֙ אֶל־מוֹצָ֣א הַמַּ֔יִם וַיַּשְׁלֶךְ־שָׁ֖ם מֶ֑לַח וַיֹּ֜אמֶר כֹּֽה־אָמַ֣ר יְהוָ֗ה רִפִּ֙אתִי֙ לַמַּ֣יִם הָאֵ֔לֶּה לֹֽא־יִהְיֶ֥ה מִשָּׁ֛ם ע֖וֹד מָ֥וֶת וּמְשַׁכָּֽלֶת׃
Então saiu ele ao manancial das águas e, deitando sal nele, disse: Assim diz o SENHOR: Sarei estas águas; não mais sairá delas morte nem esterilidade.
Rashi on II Kings
To the source of the water. From where the water flowed.
Rashi on II Kings
And threw salt there. Now is not salt a thing that spoils water? This was a miracle within a miracle.20This miracle was similar to the one performed by Moshe in Marah. After traveling through the wilderness for three days without water, the Bnei Yisroel finally came to a spring only to find the water undrinkably bitter. Moshe threw a tree whose wood was bitter into the water and the water became sweet and drinkable. See Shemos 15:22-26.—Radak