It should be borne in mind that the transporting capacity of running water varies as the sixth power of its velocity. Therefore when a stream doubles its velocity it is competent to move particles sixty-four times greater than before. If its rate of flow is trebled, its transporting power is increased seven hundred and twenty-nine times. This goes to explain the frightful havoc of streams at times of flood. The tributary streams of the Colorado come from arid regions and from the deserts, and are subject to sudden violent cloud-bursts and enormous floods. Though these are of short duration, they are of tremendous force. Earthy matter, rocky débris, and ofttimes hundreds of trees are swept along by the waters that rush in from side cañons like an awful avalanche. Lodged driftwood over one hundred feet above normal river-level tells of the magnitude of these wild floods. Where a stream has all the load of any given degree of fineness that it is capable of carrying, the entire energy of the descending water is consumed by the transportation of the water and its burden, so that none is applied to erosion. If it has an excess of load, its velocity is thereby lessened and its power to transport is diminished; consequently a part of its load is dropped. If it has less than a full load, it is in a condition to receive more, which it eagerly does. Thereby its bed is swept clean, and then only does erosion become possible. Thus it is seen that the work of transportation may at times monopolize the entire energy of a stream to the exclusion of erosion; or the two works may be carried forward at the same time. The rapidity of erosion depends upon the hardness, size, and number of the fragments in the flowing water, upon the durability of the stream-bed, and upon the velocity of the current, the element of velocity being of double importance, since it determines not only the size but the speed of the particle with which it works. Transportation is favored by an increased water-supply as much as by increased declivity, because when a stream increases in volume the increase in its velocity outruns the increase in volume, and its transporting power is correspondingly augmented. It is due to this that a stream which is subject to floods--periodical or otherwise--has a much greater transporting power than it could possess were its total water-supply evenly distributed throughout the year. During one period of volcanic activity the focus of lava-flows into the cañon was at Lava Falls. A number of lava-streams burst directly into the cañon through the walls, while several flows poured their fiery floods over the brink. What a wild and spectacular condition existed while the river, deep in the cañon, received these tributaries of liquid fire! When the flow ceased, the cañon for sixty miles was filled with lava to the depth of about five hundred feet. The lava cooled, and in time was eroded away. The records of this spectacular story are still easily read.