Isaiah 42:15

אַחֲרִיב הָרִים וּגְבָעוֹת וְכָל־עֶשְׂבָּם אוֹבִישׁ וְשַׂמְתִּי נְהָרוֹת לָאִיִּים וַאֲגַמִּים אוֹבִישׁ׃

15 I will lay waste mountains and hills and dry up all their plants; I will turn the rivers into sandbars and I will dry up the pools.

אַחֲרִיב הָרִים וּגְבָעוֹת I will lay waste mountains and hills אַחֲרִיב hifil (causative) imperfect 1 sg. of the stative verb חָרֵב, "be dry." The mountains and hills are anarthrous and indefinite; they are to be taken as generic concepts. In chapter 40, God called for the people to make the mountains and hills level; here the Lord himself demolishes them as he comes since the people have not made the way for him. The accent under אַחֲרִ֤יב is mahpak (mehuppak), which is one of the usual servi for pashta (mahpak only serves pashta); especially when it accents the verb prior to its object. It occurs again in the same construction (verb/mahpak + object/pashta) in the second colon with וְשַׂמְתִּ֤י.

וְכָל־עֶשְׂבָּם אוֹבִישׁ all their plants I will dry up; עֵ֫שֶׂב is vegetation, or even a way of saying "vegetables" the way we would when talking about food. Here and later in the verse אוֹבִישׁ (perhaps some wordplay with עֵ֫שֶׂב?) is the hifil 1st singular imperfect of the stative verb יָבֵשׁ, "be dry," so "cause to be dry" or "dry up" is the causative hifil form.

וְשַׂמְתִּי נְהָרוֹת לָאִיִּים I will turn the rivers into sandbars. None of these words poses any problem with their forms, but what does "I will set streams to islands" mean? נָהָר can't really mean anything other than "stream, river," but אִי (Isaiah uses the singular form in 23:2) can be a coast, a border, a region, an island, or a bank. The "islands" in a river are usually not large, and in a desert region they tend to be made either of stone or sand. Young prefers to take the object as "islands" rather than my "sandbars," and the phrase is quite frankly difficult.

The Mp note for לָאִיִּים indicates that it occurs just twice where the lamed is written with qames (Isaiah 42:15; 59:18). Of minor interest here is that the full Mm notation (not included in BHS because of the limitations of space in the margin) is ב֗ קמצין, revealing the Aramaic plural form of qames: קָמֶצִין. The more important function of the Mp note is that it deftly negates the proposed emendations of the critical apparatus and proves that although difficult, the reading is ancient and verified as the one the Masoretes observed.

וַאֲגַמִּים אוֹבִישׁ and I will dry up the pools. וַאֲגַמִּים is the plural of אֲגַם "muddy pond, swamp."

The picture in this strange verse is that with the coming of the Lord, nature will be reversed. Physical blessings will come to an end, as with the Last Day when God's creation will be unmade: Mountains and hills laid low (again, this was the language of repentance in chapter 40), plants withering, rivers becoming something else--something that makes them rivers no more. In the verse to follow, more changes will come, but changes outside of the natural realm.