Courtesy The Cincinnati Enquirer . With enough water, it must rise above its banks and flood. The faster water runs from the watershed into the river, the higher a flood will be. Deforestation plays several roles in the flooding equation because trees prevent sediment runoff and forests hold and use more water than farms or grasslands. Leaves reduce raindrop impact, and gentler rain causes less erosion.
Tree roots absorb water from the soil, making the soil drier and able to store more rainwater.
Tree roots hold the soil in place, reducing the movement of sediment that can shrink river channels downstream. The loss of trees played a major role in the huge Yangtze flood of 1998, says Janet Abramovitz of Worldwatch, who observes that the Yangtze watershed had lost 85 percent of its forest cover in the past few decades. In terms of flooding, the unimpeded raindrop impact on bare ground leads to heavy erosion and quick runoff. In larger watersheds, at least in the Pacific Northwest, logging tends to have less impact on peak flows, because only a small portion has been recently clear-cut, she adds. Vast amounts of eroded soil wind up in river beds, shrinking channels and the river's ability to carry water without flooding.