The extent of the Salt Lake basin is about 28,500 square miles. The western portion, amounting to 12,500 miles, sends no water to the lake, yielding all its rainfall to evaporation within its own limits. The remaining 16,000 miles includes both plains and mountains, and its tribute is unequal. To supply 66¹⁄₂ inches annually to the whole area of the lake, 2,125 miles, it must yield a sheet of water with an average thickness of 8.83 inches. In former times, when the lake had an area of only 1,820 miles, the yield of the same area was 7.43 inches. The advance from 7.43 to 8.83, or the addition of 1 inch and 4 tenths to the mean outflow of the district, is the phenomenon to be accounted for. All the water that is precipitated within the district as rain or snow returns eventually to the air, but different portions are returned in different ways. Of the snow, a portion is melted and a portion is evaporated without melting. Of the melted snow and the rain, a part is absorbed by vegetation and soil, and is afterward reabsorbed by the air; another part runs from the surface in rills, and a third part sinks into the underlying formations and afterward emerges in springs. The streams which arise from springs and rills are again divided. Part of the water is evaporated from the surfaces of the streams and of fresh water lakes interrupting their courses. Another part enters the adjacent porous soils, and either meets in them the air by which it is slowly absorbed, or else so saturates them as to produce marshes from which evaporation progresses rapidly at the surface. The remainder flows to Great Salt Lake, and is in time evaporated from its surface. The lesser portion of the precipitation enters the lake; the greater is intercepted on the way and turned back to the air. Whatever man has done to clear the way for the flowing water has diminished local evaporation and helped to fill the lake. Whatever he has done to increase local evaporation has tended to empty the lake. The white man has modified the conditions of drainage, first, by the cultivation of the soil; second, by the raising of herds; and, third, by the cutting of trees. 1. By plowing the earth the farmer has rendered it more porous and absorbent, so that a smaller percentage of the passing shower runs off. He has destroyed the native vegetation, and replaced it by another that may or may not increase the local evaporation; but this is of little moment, because his operations have been conducted on gentle slopes which in their natural condition contributed very little to the streams. It is of greater import that he has diverted water already accumulated in streams, and for the purposes of irrigation has spread it broadly upon the land, whence it is absorbed by the air. In this way he has diminished the inflow of the lake.