"But they turned away" from what the messengers brought to them, "so We sent upon them the flood of the Arim." The Arim is a dam (jisr) by which water is restrained; it was a barrier constructed in a place in the valley where water would collect.
Mujahid said: That flood which Allah sent upon them from the Arim was red water, which Allah brought from wherever He willed, and it split the dam and demolished it. It dug into the belly of the valley, away from the two gardens, so they were elevated and the water receded from them, and they dried up. He said: "And We replaced their two gardens with two gardens of bitter fruit," meaning: fruit, "the khamt" (arak-bush), "and the athl" (tamarisk).
Muhammad said: The athl is similar to the tarfa' (tamarisk). Linguists have differed regarding the lengthening or shortening of the word "tarfa'," and most of them favor lengthening it.