Isaiah 41:18

אֶפְתַּח עַל־שְׁפָיִים נְהָרוֹת וּבְתוֹךְ בְּקָעוֹת מַעְיָנוֹת אָשִׂים מִדְבָּר לַאֲגַם־מַיִם וְאֶרֶץ צִיָּה לְמוֹצָאֵי מָיִם׃

18 I will open rivers on the bare hills, and fountains in the midst of the valleys; I will make the desert into a pool of water, and the dry land into springs of water.

אֶפְתַּח עַל־שְׁפָיִים נְהָרוֹת I will open rivers on the bare hills, אֶפְתַּח is the 1st sg qal imperfect of פָּתַח, "open." It is also the verb used for uncorking a water-skin (Judges 4:19), and the picture here might be of God unstopping the source of the rivers so that the water comes gushing down in spring floods. According ot the Mp note, the adjective שְׁפָיִים "bare" occurs three times written out plene (Isaiah 41:18; 49:9 and Jer. 3:21). נְהָרוֹת are large rivers that never run dry like the wadis (נַחַל) of Canaan.

וּבְתוֹךְ בְּקָעוֹת מַעְיָנוֹת and fountains in the midst of the valleys; The -בְ of וּבְתוֹךְ parallels עַל־ above. The word itself is the construct of תָּוֶךְ "midst, middle." בִּקְעָה "valley" is especially a broad valley with "shallow walls" (Holladay) such as the "plain" at Shinar in Genesis 11:2. It frequently occurs in compound names. A מַעְיָן is a spring or fountain; the form occurs in Prov. 8:24 in the context of Wisdom's eternal nature, "When there were no springs abounding with water...I was given birth."

אָשִׂים מִדְבָּר לַאֲגַם־מַיִם I will make the desert into a pool of water, אָשִׂים qal imperfect 1 sg from שִׂים "put, place." An אֲגַם is a muddy pool, or any pool or marsh. The Mp note tells us that this word occurs just three times. The Even-Shoshan concordance lists two forms (Isaiah 41:18; Psalm 114:8) and questions the third (אַגְמוֹן) in Job 40:26, although there is an Mp note there also indicating "three occurrences." The Job Mp note also says that the three are written defectively "in this language," but it is also marked "sub loco" at that place.

וְאֶרֶץ צִיָּה לְמוֹצָאֵי מָיִם and the dry land into springs of water. צִיָּה is a feminine adjective meaning "dry," but more frequently occurs in this full phrase אֶרֶץ צִיָּה "dry land" as a parallel synonym for desert (Psalm 63:2; 107:35; Isaiah 41:18; 53:2; Jer. 2:6; 51:43; Ezek. 19:13; Hos. 2:5; Joel 2:20). A מוֹצָא is an "outlet," such as a spring in the ground. It also means the "outlet" for the sun in the morning (the east). These last two phrases echo the words of Psalm 114:8, although the words are not identical: "(God) turned the rock into a pool (אֲגַם), the hard rock into springs of water."