Because water is the heavyweight champion of heat storage. Water stores about 4.2 times the amount of energy than air per unit weight. Water is also about 800 times denser than air. That means the same volume of water can store about 3,400 times the energy of the equivalent volume of air. Hot water also evaporates more readily into air, and the amount of water air can hold increases exponentially with temperature.
On July 8, New Mexico's Rio Ruidoso unbound from its banks for the second year in a row and swelled to 20 times its typical knee-high depth. The cascade of water roared like a train, Kathy Papasan, a longtime resident on the river's edge, told me, and dark waves battered her porch. She and her husband had to flee uphill to a neighbor's house.