A Lakebed Ledger
The Zahara-El Gastor reservoir, an artificial lake created by a dam on the Guadalete River, appears as a placid body of turquoise water. Its primary purpose since construction finished in 1992 has been to store water for the irrigation of surrounding agricultural lands. The dam and its reservoir, with a total capacity of 223 cubic hectometers, are situated within the Sierra de Grazalema Natural Park, a UNESCO Biosphere Reserve. This location is important. The park's limestone karst landscapes and the area's Mediterranean climate—characterized by infrequent but heavy rainfall—mean that water runoff carries a significant load of suspended particles.
Once this runoff enters the calm waters of the reservoir, the flow slows dramatically, and the suspended materials settle to the bottom. This process turns the reservoir into a massive sediment trap. Year after year, new layers of silt, clay, pollen, and organic matter accumulate on the lakebed. Scientists can extract core samples from these deep layers of mud. Each distinct layer, or lamina, corresponds to a specific period, often with seasonal resolution. By analyzing the physical, chemical, and biological contents of these layers, researchers can reconstruct a detailed history of the surrounding environment that extends back to the reservoir's creation.