https://en.m.wikipedia.org/wiki/Drainage_basin HEIGHT 400 Drainage Basin Wikipedia

origin site 77 captures 29 May 2000 - 4 Apr 2019 DEC JAN APR Previous capture 21 Next capture 2010 2012 2013 About this capture What is a watershed and why should I care? 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 terms "basin, drainage basin, watershed, subwatershed, subshed" cause much confusion for those just becoming familiar with watershed issues. This is because they are used almost interchangeably when, in fact, they are not interchangeable. The following excerpts clarify this issue. 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 land surface or to the system of LAND DRAINAGE introduced by human activity. Natural drainage of the land surface 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 rainfall -- 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: Local examples 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: B17 B16 and B17 Christina Basin Delaware River Basin Relief Map A Relief Map B Air Photo of B17 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. How has the WRA delineated the watersheds illustrated on this site? 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: ...the highest nearby hills, ridges, etc. Connect the points (following ridge lines, crossing slopes at right angles to contour lines) until the perimeter of the watershed has been outlined" Depending on the level of detail needed, field checking of the boundary is not only useful but also helpful to gain experience in judgment regarding location of the highest points. Field confirmation is also useful to facilitate identification of recent anthropogenic alterations (ditches, dikes, roads, etc.) not visible based on the topographic map, which could significantly alter the direction of water flow and thus impact the watershed boundary. Detailed approaches (Ammann and Stone, 1991, and Williams, 1992) are available to provide further refinement and guidance (Reimold, Robert J and Robert Singer. 1998. "Watershed Tool Kit" In Watershed Management: Practices, Policies, and Coordination. McGraw-Hill: New York. 40). 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. Why should I care? 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: Communities across the nation are finding that their water resources are degrading in response to growth and development. They are also discovering that they can only protect these local water resources by thinking on a watershed-level. While the settings and resource issues that drive local watershed protection are diverse, communities often find that many of the same tools and techniques appear to work in every watershed. Watersheds are important to any community because they embody our sense of place in the landscape, and their waters are important in our daily life. Communities quickly find many reasons to protect local watersheds-whether it is because of economic benefits, recreation, flood prevention, scenery or the overall quality of life. Different groups of people often have their own unique rationale for protecting watersheds. Some may place a high value on the aquatic biological community living in these waters, while others will be more concerned about reducing stream channel erosion to the real estate in their back yard. Regardless of the reasons, it is clear that most communities now recognize the value of local watershed protection [Center for Watershed Protection (a good Web site for learning about preservation and restoration)]. Increasingly, State and Tribal water resource professionals are turning to watershed management as a means for achieving greater results from their programs. Why? Because managing water resource programs on a watershed basis makes good sense -- environmentally, financially, and socially. 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). If you live in the Christina Basin, the watershed may be the source of drinking water for you and your family. Streams and wells in the Basin provide 75 percent of the drinking water for New Castle County, Delaware and 40 percent of the drinking water for Chester County, Pennsylvania. Water suppliers provide up to 100 million gallons to 0.5 million people in the watershed. 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). Now that it is clear what watersheds are and why they are important, you will be able to utilize this Web site to help you contribute to the preservation and restoration of your watershed. By assuming a watershed identity and participating in watershed management you can change your world for the better. Remember, informed public participation is grounded in education, and is absolutely critical for providing the impetus for government action and funding opportunities. In this context the public can play a fundamental role in nourishing an escalating cycle of public participation in watershed preservation and restoration: Data Courtesy of the University of Delaware Water Resources 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 Back to CBWQMSH