The banks of the Amazon are subject to baneful forces. Variable flood levels and strong tides, which can be felt 800 kilometers inland, can severely erode the banks. This erosion can cause terra cahides (mighty rock falls). When an earthen retaining wall becomes weak, a large section of it can rip free, bringing down huge amounts of dirt, rock, and trees with a thunderous clamor. A really big and scary flood wave rolls across the river, bounces off the other side, and returns to crash against the bank. The fragile bank can then be worn away and weakened again, and, as anyone who has ever experienced mother nature's wrath could surmise about the obvious consequences, more land breaks free and plunges into the river with another mighty roar.