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In the Middle East, floods in the desert are a frequent occurrence because of the topography of the land. The mountains to the north of the Negev and to the west of the Judea Wilderness get significant amounts of rainfall during the short rainy season. The soil of the mountains cannot absorb this amount of water, so it runs off into the arid wilderness.
This photograph shows the effects of floods in the desert. Though the walls of this wadi are especially steep, the tracks made by rushing water are clearly seen. The wilderness has many of these dry riverbeds, which become larger as they get nearer the Dead Sea.
Floodwaters leave behind sand and gravel. Jesus' story of the wise man who built his house on the rock and the foolish man who built his house on the sand (Matt. 7:24-27) is probably based on this phenomenon. According to the parable, a man built his house on the sand, in a flood zone. The rains fell, the floods came, and the house was destroyed. Another man build on the rock, out of the flood area, and the rains, though severe, had no impact on his house. Imagine someone building in the wadi pictured here. Jesus' audience must have chuckled at this story.
When a flood does occur in the desert, it is amazing to behold. The sky is clear, the sun is shining, and suddenly a wall of water roars through the narrow canyon. Anyone unfortunate enough to be in the wadi is washed away, an all-too-frequent occurrence for shepherds and sheep in this region.