Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 76

Drainage density
Lower groundwater flow volumes
-Underlain by silty clay
-High runoff potential
-High drainage density
Higher groundwater flow volumes
-Underlain by porous material
-Lower runoff potential
-Lower drainage density
FIGURE 3.12
Stream drainage densities based on differences in geology.
erosion. In combination with overland flow, seepage erosion contributes to the initiation
and growth of channel networks (Dunne 1990; Abrams et al. 2009). Many of the United
States' urban watersheds of glacial-lacustrine origin have higher drainage densities in
sandy soils, and this pattern could be related to the initiation of channels by groundwater.
Although the exact causes of channel density have not been resolved (Abrams et al. 2009),
the regional geology provides a good starting point for gaining important information
about the surface streams.
As this discussion reveals, there is a close relationship between surface water and
groundwater. At many locations, surface water recharges groundwater and, in turn,
groundwater discharges to surface water, but this is only one part of a bigger picture. We
now turn our attention to groundwater.
3.4 Groundwater in Watersheds
Groundwater is defined as any water beneath the surface of the ground (Heath 1983) and
is often described in terms of its occurrence in an aquifer. An aquifer is a naturally occur-
ring mappable geologic unit(s) composed of water-saturated porous media capable of stor-
ing and transmitting significant quantities of water under ordinary conditions (modified
from Freeze and Cherry 1979). For example, the unconsolidated geologic material compos-
ing many aquifers consists of sands and gravels, and these formations are deemed aqui-
fers because they meet the requirements just defined. Unconsolidated geologic material
composed of silt and clay can also make the grade for aquifer status, but sometimes these
finer-grained materials do not transmit water quickly enough to fulfill the definition. In
fact, clay layers have such a low permeability that we will refer to them as confining layers
that retard the downward flow of groundwater.
Early civilizations such as the Persians and Romans recognized the importance of
groundwater as a source of drinking water and as a supplement to precipitation for crop
irrigation. The presence of water in the subsurface is influenced by fundamental geologic
processes, including the subsurface rock type and the character of the tectonic activity
influencing the region (e.g., folding or faulting). Today, the distribution and abundance of