Collapse: How Societies Choose to Fail or Survive

by Jared Diamond

Here is how a saline seep arises. Eastern Montana has lots of water-soluble salts (especially sodium, calcium, and magnesium sulfates) present as components of the rocks and soils themselves, and also trapped in marine deposits (because much of the region used to be ocean). Below the soil zone is a layer of bedrock (shale, sandstone, or coal) that has low permeability to water. In dry eastern Montana environments covered with native vegetation, almost all rain that falls is promptly taken up by the vegetation’s roots and transpired back into the atmosphere, leaving the soil below the root layer dry. However, when a farmer clears the native vegetation to practice crop-and-fallow agriculture, in which an annual crop like wheat is grown during one year and the land is left fallow the next year, there are no plant roots to take up rainwater falling in the fallow year. That rainwater accumulates in the soil, waterlogs it below the root layer, and dissolves salts that then rise into the root zone as the water table rises. Because of the impermeable underlying bedrock, the salty water doesn’t drain deeply into the ground but emerges somewhere downhill nearby as a saline seep. The result is that crops grow more poorly or not at all, both in the uphill area where the problem arises and in the downhill area where the seep emerges.