The geology and geography of Colorado is the foundation of the state’s water resources. Colorado’s complex geologic history has divided the state into fractured, crystalline rock mountain ranges; deep basins and fault-bounded valleys in which thousands of feet of sediment accumulated; and areas of relatively undisturbed flat-lying sedimentary deposits. The various aquifers throughout Colorado can be categorized by the associated geologic deposits and rocks of which they are composed. Colorado’s principal aquifers are categorized into the following:
- unconsolidated Quaternary age alluvial aquifers associated with the major river systems;
- poorly consolidated or unconsolidated sediments;
- consolidated sedimentary rock aquifers; and
- volcanic and crystalline rock aquifers.
For the purposes of this atlas, specific chapters are dedicated to discussions of Colorado’s Quaternary-age alluvial deposits, the state’s major sedimentary rock aquifers or aquifer systems, and the aquifers that occupy the central one-third or mountainous regions of the state. Alluvial deposits associated with major river systems consist of unconsolidated silt, sand, and gravel that have been deposited during recent geologic time by water transport as sorted or semi-sorted sediment. The alluvial aquifers associated with these deposits are unconfined and contain ground water stored in stream-deposited unconsolidated sediment along river valleys. Ground water in alluvial aquifers usually interacts with surface water of the stream system, and ground-water levels may exhibit seasonal variation in response to surface-water flow. Chapter 5 discusses the hydrologic characteristics of the alluvial aquifers associated with Colorado’s major river watersheds. The distribution of these alluvial deposits is illustrated in Figure 4.1.