What is the theme in The Control of Nature by John McPhee?

The main theme explores how solid earth flows. An irony in the expression on solid ground is that forming solid ground requires a liquid. The Mississippi River carries silt and sediment that it deposits in its riverbed. The riverbed builds up over time. The river forms levees and leaves new soil when it overflows its banks. Centuries of depositing this watery residue forms the Louisiana delta and New Orleans land mass. Flooding enables the sedimentation process to add solid land mass to the area. Water that flows downriver from the north begins as solid ice and snow in the far northern Mississippi.