Prior to its agricultural development in historic times, the Rainwater Basin wetland region was characterized by numerous playa wetlands, "Rainwater Basins", numbering in the thousands. The shallow depressions, in which these wetlands occur are lined with a nearly impervious layer of clayey soil, a claypan, that prohibits surface water from penetrating the subsoil. As a result, Rainwater Basins are not naturally influenced by the water table and the sole source of water is run-off in the form of rain, snow and, currently, drainage from crop irrigation. Because the primary source of water for these wetlands is precipitation, they annually vary in depth, expanse and seasonality due to changes in precipitation regimes and are called "Rainwater Basins".