Hydrology and Physiography of Groundwater and Wetland Habitats - Freshwater Ecology: Concepts and Environmental Applications - page 55

Where the groundwater impinges on the surface, a stream, lake, or
wetland forms (Fig. 4.5). The hyporheic zone represents the interface
through which materials are exchanged between surface and groundwa-
ter. This zone may include interstitial water of sediments below lakes and
wetlands, gravel bars in rivers, sand below streams, and many other ben-
thic habitats in aquatic systems. As with any habitat classification, the
distinction between the hyporheic zone and groundwater is unclear be-
cause the zone is transitional and varies over space and time and it de-
pends on whether material transport or habitats of organisms are con-
sidered (Gibert et al., 1994). For example, hyporheic zones that are
formed by river action can be quite complex because of erosion and de-
position that naturally occur in the stream channel (Creuzé des Châtél-
liers et al., 1994). The importance of hyporheic zones has become ap-
parent to aquatic ecologists (Danielopol et al., 1994; Gounot, 1994;
Allan, 1995; Stanley and Jones, 2000), and methods have been developed
to quantify their connections with surface water (Harvey and Wagner,
2000).
Groundwater is located worldwide, but the depth of the aquifer be-
low the surface and the amount in the aquifer can vary across the land-
scape. In porous substrata, wells can yield a large amount of water. In
some areas, such as those underlain by solid rock, groundwater yields can
be very low. Examination of the distribution of large aquifers demon-
strates heterogeneity in the types of aquifers found in the United States
FIGURE 4.7 Principal large aquifers used by humans of the conterminous United States.
1, Semiconsolidated sand aquifers; 2, unconsolidated gravel and sand aquifers; 3, basaltic and
volcanic rock aquifers; 4, sandstone and carbonate rock aquifers; 5, sandstone aquifers; and
6, carbonate rock aquifers. Sand and gravel aquifers north of the dark line are mainly glacial
deposits. Localized aquifers may occur in areas that are not shaded (image courtesy of the
U.S. Geological Survey).