Adam Clarke on Job 6:15

Have dealt deceitfully as a brook - There is probably an allusion here to those land torrents which make a sudden appearance, and as suddenly vanish; being produced by the rains that fall upon the mountains during the rainy season, and are soon absorbed by the thirsty sands over which they run. At first they seem to promise a permanent stream, and are noticed with delight by the people, who fill their tanks or reservoirs from their waters; but sometimes they are so large and rapid as to carry every thing before them: and then suddenly fail, so that there is no time to fill the tanks. The approach of Job's friends promised much of sympathy and compassion; his expectations were raised: but their conduct soon convinced him that they were physicians of no value; therefore he compares them to the deceitful torrents that soon pass away.