Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 22

• Are difficult to access because many desirable study locations are on private
property.
• Have been disturbed and modified due to development.
• Are typically located on unconsolidated sediments of Quaternary age and were
historically considered overburden and not as scientifically valuable as the bed-
rock located beneath the unconsolidated deposits.
• Are very complex and time consuming to study.
• Involve issues that may take decades to be noticed by the general community as
compared with other more sudden types of geologic hazards such as earthquakes,
tsunamis, and volcanic eruptions.
In addition, the geology of most urban regions generally goes unnoticed by most residents
and even the scientific community because it is not visually spectacular like the geology
seen in our national parks. Most residents would be more inclined to learn about a region's
geology and its underlying processes if their area was prone to earthquakes, such as parts
of California, or volcanic eruptions, such as the Pacific Northwest or Hawaii. The geol-
ogy beneath most urban regions affects our day-to-day lives in more subtle ways than an
earthquake or volcanic eruption; however, its ultimate impact on the quality of our lives
could be just as significant.
Watersheds functioned well before humans began to alter their natural flows of water;
and now many are in a distressed state. Understanding what was once at a given location
(site) and how it came into existence are therefore prerequisites for correctly analyzing
the current problem, and prescribing an effective and sustainable remediation strategy.
As more and more sites of human interference are analyzed, the overall picture of the
linkages between the watershed's geology, biology, geography, and human interference
becomes clearer. As more of these linkages are made, the geographic extent and impacts
of contamination in different geological environments become more evident, appropriate
remediation efforts can be more effectively implemented, and the methods for sustainable
redevelopment and new development refined.
Our efforts in this chapter concentrate on the sedimentary processes influencing urban
areas and their watersheds. We begin with the formation of rocks and the geologic princi-
ples governing sedimentary formations and their arrangements (uniformity, stratigraphy,
and unconformities), then move on to the processes involved in determining sedimentary
rock composition and transport, and the environments created from the combination of
the rock characteristics and their depositional processes. The chapter concludes with a
discussion of natural and anthropogenic disturbance.
2.2 Geological Processes Affecting Urban Watersheds
Geological processes affecting urban watersheds generally take place at or very near the
surface and are primarily sedimentary-type processes, which include processes involv-
ing liquid and solid water (ice), gravity, and tectonic activity. Sedimentary deposits are
defined as the accumulation of natural materials and sediments formed at or near the
surface of the Earth at ordinary temperatures and pressures (Flint and Skinner 1974).
Geological processes of sedimentary origin usually occur gradually over long periods