STORMWATER HYDROLOGY AND QUALITY - Low Impact Development and Sustainable Stormwater Management - page 35

Wetlands
There are many potential land development sites that are constrained by wetlands
and other sites where an existing wetland system largely prevents development
or requires the destruction of the wetland in order to build. The most important
concept to grasp in considering this site condition is that wetlands do not exist
in a disconnected manner from the balance of the site or watershed, but are a
reflection of natural surface or subsurface drainage conditions that produce a
saturated land surface for at least a portion of the year. In simple terms, they
generally are places where the surface-groundwater connection is most apparent,
and in many ecosystems where rainfall is emerging from the groundwater.
There can also be geologic conditions that result in closed surface depressions
where wetlands form on the surface, especially in regions of the country that
experienced glaciers during the last Ice Age (ending about 25,000 years ago),
where the receding glaciers (Figure 2-1) left behind blocks of ice to form “ket-
tle holes” or other depressions in the landscape. The landscapes of the upper
Midwest, in states such as Minnesota and Wisconsin, contain rich ecologies of
wetlands that are internally drained and reflect the regional water table without
a surface outlet.
For most watersheds, however, the wetlands that have formed along the ripar-
ian stream or river channels, or around springs and seeps on the hillside, are
locations of groundwater discharge. Regardless of the hydrogeologic origins of a
wetland, they provide an essential component to the water balance, and in many
ways are the beginnings of the land-water continuum. They are an essential part
of the hydrologic cycle, and their loss affects the entire watershed.
Figure 2-1 The last ice age came to an end about 25,000 years ago.