A water resource region is the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units as part of the U.S. hydrologic unit system.
The South Atlantic–Gulf water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
The Upper Mississippi water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
The Lower Mississippi water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
The Missouri water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
The Ohio water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
The Tennessee water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
The Rio Grande water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
The Upper Colorado water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
The Lower Colorado water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
The Great Basin water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
The Pacific Northwest water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
The California water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey in the United States hydrologic unit system, which is used to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
The Texas–Gulf water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
The Arkansas–White–Red water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.
In order to advantage hydrologists, ecologists, and water-resource managers in the study of "water, its properties and laws, and its distribution over the earth's surface" in the United States, the United States Geological Survey created a hierarchical system of hydrologic units.
Ventura–San Gabriel Coastal water resource basin is one of three hydrologic basins within the Southern California Coastal water resource subregion and is one of approximately 2,200 water resource basins in the United States hydrologic unit system. The Ventura–San Gabriel Coastal water resource basin is a third-level subdivision of the United States hydrologic unit system.
Northern Mojave–Mono Lake water resource subregion is one of 10 water resource subregions within the California water resource region and is one of 222 water resource subregions in the federally organized United States hydrologic unit system.
Santa Ana water resource basin is one of three hydrologic basins within the Southern California Coastal water resource subregion and is one of approximately 2,200 water resource basins in the United States hydrologic unit system. The Santa Ana water resource basin is a third-level subdivision of the United States hydrologic unit system.
Laguna–San Diego Coastal water resource basin is one of three hydrologic basins within the Southern California Coastal water resource subregion and is one of approximately 2,200 water resource basins in the United States hydrologic unit system.
