Such a boy hardly need ask the conductor how far west he is if he can catch a glimpse of one of the rivers. All the rivers of the plains are alike full of yellow mud, because the soil of the plains melts at the touch of water. These are our spendthrift rivers, full to the banks at times, but most of the year desperately in need of water. It is only with the greatest effort that they can keep their places in the summer: there is just a scanty thread of water strung along a great, rambling bed of sand, to restrain Dame Nature from revoking their licenses to run and turning them into cattle-ranches. No wonder that fish refuse to have anything to do with such streams, and refuse tempting offers of free worms, free transportation, and protection from the fatal nets. Fancy trying to raise a family of little fish, and not knowing one day where water is coming from the next! Not but what there is water enough at times; only, those rivers of the great plains, like the Platte and the Kansas and the Arkansas, are so wasteful of their supply in the spring that by July they are gasping for a shower. So, part of the year they revel in luxury, and during the rest they go shabby--like shiftless people. But the irrigation engineers have lately discovered something wonderful about even these despised rivers. During the very driest seasons, when the stream is apparently quite dry, there is still a great body of water running in the sand. Like a vast sponge, the sand holds the water, yet it flows continually, just as if it were in plain sight, but more slowly of course. The volume may be estimated by the depth and breadth of the sand. One pint of it will hold three quarters of a pint of water. This is called the underground flow, and is peculiar to this class of rivers. By means of ditches this water may be brought to the surface for irrigation. Scattered among the foot-hills of the Rockies are rivers still more wilful in their habits. Instead of keeping to their duties in a methodical way, they rush their annual work through in a month or two; then they take long vacations. For months together they carry no water at all; and one may plant and build and live and sleep in their deserted beds--but beware! Without warning, they resume active business. Maybe on a Sunday, or in the middle of the night, a storm-cloud visits the mountains. There is a roar, a tearing, a crashing, and down comes a terrible wall of water, sweeping away houses and barns and people. No fishing, no boating, no swimming, no skating on those treacherous rivers; only surprise and shock and disaster! So different that they seem to belong in a different world are the great inter-mountain streams, like the Yellowstone and the Colorado.