What is a watershed?
What is a watershed? A watershed is a basic hydrological unit. In U.S. and Canadian mainstream media this word has come to be synonymous with the terms drainage basin and catchment. Thus, it has become common practice to use the term watershed loosely (if incorrectly) in order to refer to areas such as the "Chesapeake Bay Watershed." Each "watershed" has its own network of river and stream channels that drain water from and through a particular basin. The characteristics of that drainage network play a great part in determining how water moves through the basin and consequently impacts upon issues such as water quality and quantity (including flooding) in a given place. However, this surface-based watershed concept does not necessarily allow one to predict sub-surface movements of water. It is important to grasp the concept that individual drainage basins are not self-contained entities, they are pieces of a puzzle incorporated into larger surrounding watersheds that represent only a small portion of the greater hydrologic cycle:
Watersheds, or basins, drain into one another taking the form a nested
hierarchy. It is topography that primarily determines where and how
water flows from one area to the next. However, each large drainage
basin can be broken into smaller drainage basins with their own topographic
and hydrologic characteristics, these are called sub-watersheds, or subsheds
for short. The flow of water (and whatever it carries with it)
is influenced by large features such as a continental divides, but one
can also focus on drainage around an individual river. Thus, watersheds
come in all shapes and sizes. This also means that each watershed has a
sub-watershed. Understanding scale and geomorphology is of utmost
importance when studying "interconnectedness" of watersheds. In short,
watershed analysis such as that provided on this Web-site demonstrates
how what we do in our basin dramatically affects people and the environment
"downstream" regardless of administrative borders, many times over long
distances, and often on a very large scale with long lasting implications.
The Encyclopedic Dictionary of Physical Geography (Andrew Goudie; Atkinson,
BW; Gregory, Kenneth J.; Simmons, IG; Stoddart, DR; Sugden, David, eds.
1997. 3rd ed.
Blackwell Publishers, Malden: MA, 153) offers the following explanation
(all caps indicate definitions of those words are also listed in book):
| Drainage
May refer either to the natural drainage of the landsurface or to the system of LAND DRAINAGE introduced by human activity. Natural drainage of the landsurface is organized in drainage basins which are those areas in which water is concentrated and flows into the DRAINAGE NETWORK. The drainage basin is usually defined by reference to information on surface elevation, for example, from contours on topographic maps although the position of the WATERSHED on the ground surface, which is the line separating flow to one basin from that to the next, may not correspond to the PHREATIC DIVIDE beneath the surface, The pattern of natural drainage has been studied in relation to the DRAINAGE DENSITY and DRAINAGE BASIN CHARACTERISTICS which can be quantified and used in rainfal -- run-off modeling and in the interpretation of river discharge; to the nature of the drainage network including the pattern of the drainage and also the stream ORDER; and to the evolution of the drainage pattern. In the course of drainage evolution the details of the several patterns such as trellis or rectangular (see diagram for DRAINAGE NETWORK) may be related to geological structure such as the alternation of hard and soft rocks or to the presence of joints or faults. Where drainage patterns are discordant with the structure and cross folds or faults, for example, it has been suggested that either the drainage has been superimposed from a cover rock that originally occurred above the rocks at present exposed in the landscape, or the drainage was antecedent and the drainage pattern was maintained as the structures were developed by endogenetic uplift giving the folded and/or faulted structures. |
The preceding definition may seem somewhat overwhelming, even inaccessible
-- this is representative of the fact that watershed science is intricate.
The following may further clarify the matter. The Center for Watershed
Protection's Rapid Watershed Planning Handbook: A Comprehensive Guide
for Managing Urbanizing Watersheds (www.pipeline.com/~mrrunoff, 1998, 1.2
- 1.3) provides another perspective on how to classify basins:
 |
 |
While contours on a topographic map may show where water will flow, it is easy to see that picking the size of a watershed to identify is somewhat subjective. For instance, the Brandywine -- mainstem through Wilmington (B17) lies within the Brandywine Creek subwatershed, which in turn resides within the Christina Watershed, which is a part of the Delaware River Basin. Consequently, what happens in the Brandywine affects the health of both the Christina Watershed and the Delaware River Basin. So does one say that he or she lives in the Christina Watershed or the Brandywine subshed? The most accurate answer is "both." To add to the confusion, the EPA has delineated somewhat different shaped watersheds than some states. But do not be distracted by semantics and arguments concerning the finer points of watershed delineation. The point is that what happens in a given watershed has important consequences for the health of other watersheds. Watersheds fit together like pieces of an interdependent puzzle. Here are explanations and illustrations from the Christina Basin to help you understand how watersheds form a nested hierarchy:
Satellite AVHRR of Delaware River Basin A
Satellite AVHRR of Delaware River Basin B
Video clips (avi) describing watersheds
All images are referenced to the Christina Basin base map.
Digital Orthographic Quarter Quadrangle (DOQQ), a a aerial photograph shot with infrared film courtesy of the University of Delaware Spatial Analysis Lab.
Satellite data courtesy
of Ray Sterner of the Johns Hopkins Advanced Physics Laboratory from the
Color
Landform Atlas of the United States.
Now that the concept of "What is a Watershed" has been outlined, it is
important to understand how organizations such as the WRA delineate (select)
basins. This is accomplished in several ways. However, this
presentation will focus upon the use of topographic maps for delineation.
The process requires a "feel" that is gained and improved with experience.
However, the general steps are outlined below, followed by a series of
maps that illustrate an actual delineation at various scales, thus allowing
the viewer to see topographic details that drive the delineation process.
To more easily follow the following steps it may be wise to either print
the maps linked below or open them up in separate windows so that they
can be examined as each point is read. Step:
|
1. Acquire United States Geological Survey 7.5 minute series topographic map(s). This will allow one to begin the selection process. How many maps one needs depends on the scale of the basin to be delineated. For instance, if one chooses to define a catchment area for a small stream perhaps only one map will suffice. However, if the goal is to determine the major watersheds for all of northern Delaware by large streams (i.e. Brandywine, Christina, Red and White Clay Rivers) then many maps will be needed to achieve full coverage. 2. Find the stream that defines your area of interest. Begin by placing a highlighter on the spot downstream at the point where the tributary (stream) enters a higher order (larger size) stream, this will be referred to as the point of origin. In our illustrations below that point can be found in the southeast area portion of the quadrangle where Wilson Run meets the Brandywine River. 3. Next, work the highlighter up the main stream of interest (see yellow highlighter in illustrations) and trace the stream and all of the tributaries that can be seen flowing into it. 4.
In either a clockwise or counter-clockwise manner (it does not matter which)
trace the watershed boundary (divide) with a pencil from its point of origin.
This is accomplished by finding and marking:
A helpful thing to understand is that upward pointing contours are that form cone-like shapes that represent land sloping towards the wide end of the cone (see illustration noting the shape of the contour lines around tributaries). Also, when connecting points to create a boundary always connect the closed loops and highest points while at the same time staying on your stream's side of the highest ridge separating drainage areas. Finally, when in doubt, trace the contour line of the highest ridge separating your drainage area from others, when "crossing slopes at right angles to contour lines" connect hill-tops and ridges. Visualize that if you draw your divide correctly an imaginary drop of water that fell on one side of the line would drain to the next tributary, while on the other side it would fall flow into your tributary. This is admittedly complex, but observing the illustrations below will help to clarify. For more on learning to use topographic maps click here. 5. Once satisfied that the pencil tracing is accurate, trace over the pencil with a highlighter (red in the illustration below). 6. At this point the watershed boundary can be digitized and processed for integration into a geographic information system (GIS) for digital mapping. This is how watershed boundaries have been mapped for this Web site. However, phenomena such as land use and topography are normally derived from aerial photography and satellite imagery. |
All
images are cut from United States Geological Survey 7.5 minute series topographic
maps (Wilmington North Quadrangle and Kennett Square Quadrangle) which
encompass subwatershed B-16, which can be referenced via the Christina
Basin base map.
It is not enough to be told that you should care about watershed -- you need reasons why this topic deserves your attention. Here are a few reasons as presented by a variety of well respected contributors to the field of watershed science:
Because watersheds
are defined by natural hydrology, they represent the most logical basis
for managing water resources. The resource
becomes the focal
point, and managers are able to gain a more complete understanding of overall
conditions in an area and the stressors
which affect those
conditions.
Traditionally, water
quality improvements have focused on specific sources of pollution, such
as sewage discharges, or specific water
resources, such
as a river segment or wetland. While this approach may be successful in
addressing specific problems, it often fails to
address the more
subtle and chronic problems that contribute to a watershed's decline. For
example, pollution from a sewage treatment
plant might be
reduced significantly after a new technology is installed, and yet the
local river may still suffer if other factors in the
watershed, such
as habitat destruction or polluted runoff, go unaddressed. Watershed management
can offer a stronger foundation for
uncovering the
many stressors that affect a watershed. The result is management better
equipped to determine what actions are needed
to protect or restore
the resource.
North Carolina was
able to monitor nearly 40 percent more waters with the same level of effort
after monitoring was conducted on a more coordinated watershed basis.
Besides the environmental pay-off, watershed approaches can have the added
benefit of saving time and money. Whether the task is monitoring, modeling,
issuing permits, or reporting, a watershed framework offer many opportunities
to simplify and
streamline the workload. For example, synchronizing monitoring schedules
so that all monitoring within a given area
(i.e., a watershed)
occurs within the same time frame can eliminate duplicative trips and greatly
reduce travel costs. North Carolina
was able
to monitor nearly 40 percent more waters with the same level of effort
after monitoring was conducted on a more coordinated watershed basis.
Efficiency is also
increased once all agencies with natural resource responsibilities begin
to work together to improve conditions in a
watershed. In its
truest sense, watershed protection engages all partners within a watershed,
including Federal, State, Tribal and local
agencies. By coordinating
their efforts, these agencies can complement and reinforce each others'
activities, avoid duplication, and
leverage resources
to achieve greater results (United
States Environmental Protection Agency, Office of Water).
By protecting our watersheds we can protect our water supplies and the integrity of the our lands. Residents can protect the watershed by planting trees, cutting back on lawn fertilizer and pesticide use, and recycling household wastes like motor oil instead of dumping into storm drains. We can all do our part to protect our watershed (University of Delaware Water Resource Agency).
Disclaimer: The information
contained herein is intended to provide general information. While the
WRA makes
makes every effort
to confirm the accuracy of this information, it does not warrant or guarantee
information
being provided is
accurate, current or complete. The Water Resource Agency and the
University of Delaware
accept no responsibility
for damages or any losses based upon reliance on this information.
All
questions may be directed to: Attention Web site manager nminni@udel.edu
