The Quran

أَنزَلَ مِنَ السَّماءِ ماءً فَسالَت أَودِيَةٌ بِقَدَرِها فَاحتَمَلَ السَّيلُ زَبَدًا رابِيًا ۚ وَمِمّا يوقِدونَ عَلَيهِ فِي النّارِ ابتِغاءَ حِليَةٍ أَو مَتاعٍ زَبَدٌ مِثلُهُ ۚ كَذٰلِكَ يَضرِبُ اللَّهُ الحَقَّ وَالباطِلَ ۚ فَأَمَّا الزَّبَدُ فَيَذهَبُ جُفاءً ۖ وَأَمّا ما يَنفَعُ النّاسَ فَيَمكُثُ فِي الأَرضِ ۚ كَذٰلِكَ يَضرِبُ اللَّهُ الأَمثالَ

Muhammad Asad

[Whenever] He sends down water from the sky, and [once-dry] river-beds are running high¹ according to their measure, the stream carries scum on its surface;² and, likewise, from that [metal] which they smelt in the fire in order to make ornaments or utensils, [there rises] scum. In this way does God set forth the parable of truth and falsehood: for, as far as the scum is concerned, it passes away as [does all] dross; but that which is of benefit to man abides on earth. In this way does God set forth the parables

The interpolation of the adjective "once-dry" before "river-beds" (awdiyah) is necessitated by the absence of the definite article al before this noun. According to Zamakhshari, this indicates that only some of the river-beds are streaming with water while others, not affected by this particular rainfall, remain dry. It is to be borne in mind that the term wad (or wadi in popular parlance) denotes, primarily, a "water-course" or "river-bed" which is normally dry and carries water only after copious rainfalls; it is only by extension that this term is sometimes applied to an actual river.
Sc., "while the water beneath remains clear".