We called this the Grand Canyon model, and it showed many features similar to those of South Dakota Bad Lands and the Colorado River drainage. It was strictly rainfall erosion and stratification soakage and seepage. The model surface sloped ten degrees, the high divide at the top had a backslope of forty-five degrees, and everything was sprayed for two months with special hose nozzles, making during part of each day a mistlike rainfall. The steep backslope did not trench itself at all despite its steepness. This slope, on the contrary, absorbed moisture and carried the rainfall underground down the dip of the strata to add spring water to the main streams. The backslope was a “steep escarpment,” supposed in physical geography to migrate by trenching backward, but the rills never gained volume enough to cut into it. All the water volume acquired its grit for cutting from the large surfaces, which were gradually tilted in the direction of the rivers. When the complete series of experiments on erosion and sediment was published, it showed that the treelike branching of rivers is dependent on underground water surfaces; that meanders on a flood plain are partly a bubbling-up process of flood-plain soakage; that when side tributaries form by undermining, the upstream branches cut off underground water from the downstream branches; and that when a country is tilted in one direction, there is a tendency to parallel streams, separated by intervals controlled by underground water areas reached by the undermining tracery of headwater springs. This arborescence in a spray model is a regular and delicate adjustment, where a bunch of tributaries is not mere catchment of rainfall, but is the product of sheet flood in belts of underground water related to the tilt of the country. Arborescence of river drainage on a surface of flat strata, like the coastal plain of the southeastern United States, is a rhythmical pattern of exquisite design capable of reproduction and study in the laboratory. It is a mathematical forking and headward development dependent on volume of water, undermining impermeable strata along permeable ones. And after the “tree” map is formed, the bulb of branches and twigs and underground leaves of spring water holds all the downslope country in its “shadow,” so that no new rivers can form there. This is what makes our great maps of river systems. It is not haphazard. It is a vast ocean of underground water, with mountains of water and valleys of water.