8.4.2: Watershed Structure

Watershed structure includes structure of flowing waters (mainly rivers and streams with associated riverine wetlands and riparian zones), still waters (lakes and associated basin-type wetlands and shorelands), and upland areas of watersheds.

Figure \(\PageIndex{1}\): . A four-dimensional concept of watershed structure (after Ward, 1989). (USEPA Introduction to Watershed Ecology, public domain)

Flowing (Lotic) Systems

The US has more than 3.5 million miles of flowing water systems, which include springs and seeps, rivers, streams, creeks, brooks and side channels.

The Four-Dimensional Concept (Figure \(\PageIndex{1}\)) recognizes that lotic systems’ structure exists in a four-dimensional framework, as below:

Longitudinal (in an upstream and downstream direction) - Flowing water systems commonly go through structural changes en route from their source to mouth. Three zones are usually recognized – headwaters, where flow is usually lowest of any where along the system, slope is often steepest, and erosion is greater than sediment deposition; transfer zone, the middle range of the stream where slope usually flattens somewhat, more flow appears, and deposition and erosion are both significant processes; and the downstream end’s depositional zone, where flow is highest but slope is minimal and deposition of sediment significantly exceeds erosion most of the time.

Lateral (across the channel, floodplains and hillslopes) - Again, significant variation occurs among stream types, but a common pattern includes the channel, the deepest part of which is called the thalweg; low floodplains that a re flooded frequently, and higher floodplains (e.g., the 100-year or 500-year) that are rarely inundated; terraces, which are former floodplains. A four-dimensional concept of watershed structure (after Ward, 1989) downcutting stream no longer floods; and hillslopes or other upland areas extending up-gradient to the watershed boundary.

Vertical (surface waters, ground water and their interactions) - It is always important to recognize that water bodies are not purely surface features; rivers and streams constantly interact with groundwater aquifers and exchange water, chemicals, and even organisms. Over its entire length, a stream often varies between influent reaches where surface water leaks downward into the aquifer, and effluent reaches where the stream receives additional water from the aquifer.

Temporal (through time, from temporary response to evolutionary change) - The dimension of time is important because rivers and streams are perpetually changing. Structure as described in the other three dimensions above should never be considered permanent, and watershed managers should always think of structure not just as what is there now, but in terms of the structural changes in progress and their rates of occurrence.

Recognition of different types of streams and rivers is mostly reliant on channel form and function. For more on stream and river categories see the Stream Channel Classification box on the following page.

Structure in Upland Areas of Watersheds

Figure \(\PageIndex{2}\): . Aerial views reveal the complexity of landscape pattern, which implies many traits of a watershed’s ecological functions. (USEPA Introduction to Watershed Ecology, public domain)

The physical form of the uplands in watersheds can vary greatly, in ways beyond the scope of this discussion. Here we focus only on the distribution of and variations in vegetation and land use, which together create the element of watershed structure called landscape pattern (Figure \(\PageIndex{2}\)). Vegetation and land use patterns in watersheds are known to have many significant influences on the condition of the water bodies they drain into; this topic is explored in greater detail in the Academy 2000 module on Watershed Change (http://www.epa.gov/watertrain/a gents).

Figure \(\PageIndex{3}\): Useful units for describing landscape structure and pattern. (USEPA Introduction to Watershed Ecology, public domain)

Landscape patterns. Landscape ecology offers a simple set of concepts and terms for identifying basic landscape patterns: matrix, patch, and mosaic (Figure \(\PageIndex{3}\)). The ecological term matrix refers to the dominant (> 60 percent) land cover, while a patch is a nonlinear area that is less abundant and different from the matrix. A mosaic is a collection of different patches comprising an area where there is no dominant matrix. Various patch types have been described in the Examples of Different Patch Types box below. Basically, the most obvious landscape patterns are formed by combinations of native vegetation communities, unvegetated areas, and land use patterns.

Landscape pattern change. The individual patches in a landscape can change, and so can the entire landscape change in pattern and/or composition. Disturbances and various landscape processes maintain a constant dynamic, referred to as a shifting mosaic. Some landscapes remain in a “dynamic equilibrium” and, although changing steadily from place to place, retain an important quality called mosaic stability. A well-managed forestry operation, for example, would exhibit over the long term a constantly shifting set of locations where mature forest occurred, but at the same time sustains the relative proportions of forested and nonforested land in the area. Or, a landscape may evolve toward a new type of pattern and composition (e.g., via timber clearcutting, suburban sprawl, abandonment and succession of agricultural lands back to forest, or landscape change due to disease, fire, or global warming). It is always important, when analyzing landscape pattern and landscape change, to remain aware that the spatial resolution of your information (how small a landscape feature you can detect) may or may not be sufficient to detect all the landscape changes of possible significance that may be occurring.

Vegetational patterns. Upland vegetation structure varies spatially, following various biogeographical patterns based on climate, physiography, soils, disturbance regimes, and their interactions (Figure \(\PageIndex{4}\)). Vegetation communities are areas where a few species of plants dominate and establish a characteristic form or structure, within which a potentially large number of less abundant organisms also exist. Nationwide, there are hundreds of vegetation community types; the Society of American Foresters recognizes over 80 forest types alone (SAF, 1980). As a first step in analyzing vegetational patterns, it is easier to recognize a few generalized upland vegetation types based on their growth form, including:

• Forests (deciduous, evergreen and mixed)
• Shrublands
• Grasslands
• Forbs (broad-leaved herbs)

Figure \(\PageIndex{4}\):Examples of matrix, patch, and corridor. (USEPA Introduction to Watershed Ecology, public domain)

These categories are commonly found on land cover maps likely to be available in the GIS data for most watersheds, and can be consulted to give a general sense of vegetation patterns in the watershed. Human activity has carved up and fragmented many of the natural vegetation patterns that formerly covered our watersheds. Without human influence, however, vegetation patterns would not be uniform due to different vegetation communities arising from different environmental conditions (e.g. variations in moisture and temperature due to slope and aspect) and events (e.g., fire, pest outbreak). In the West, the “rain shadow” is a common, basic example of how vegetation varies with physical position.

Example: Southern Sierra As moisture-laden air is pushed east, it begins to slowly rise as it reaches the graduallysloping foothills of the southern Sierra. As the air rises, pressure increases, and moisture in the air begins to condense. As the air continues eastward, it continues upward to the crest of the range, and moisture within the air is seemingly wrung-out, much like a sponge. The eastern slope of the Sierra is quite dramatic, topographically. Indeed, Mt. Whitney, the highest point in the coterminous United States, is just a matter of miles from North America’s lowest point (Death Valley, CA). Much of the eastern slope is quite dry, since the air is now deprived of much of its moisture.

Land-use patterns. Increasingly, the landscape structure and pattern we see is the result of widespread human activity. In all fields of environmental management including watershed management, analysis of land use types, patterns, and trends is commonplace. Because multiple uses occur in many locations and some land uses are not in themselves a visible landscape feature, mappers often use term land cover to describe the delineation of landscape structure and pattern formed by the dominant land uses and remaining vegetation communities. Some common land cover categories (indicating land uses within the areas) include:

• urban land (residential, commercial, industrial, mixed)
• agriculture (row crops, field crops, pasture)
• transportation (roads, railroads, airports)
• rangelands
• silviculture
• mining/extractive areas

Like vegetation patterns, the land use patterns in a watershed can be studied through GIS data or maps. Human-dominated landscapes, just as natural landscapes, are shifting mosaics that often progress through a series of changes in what is dominant.

Example: Rural Upstate New York Pre-settlement landscapes in upstate New York were probably almost entirely forested. By the mid to late 19th century, clear-cutting had removed the forest from most areas. In the first part of the 20th century, cropland and pasture became widespread and dominated 70% or more of the landscape, with remnant scattered forest patches, wetlands and lakes. By the second half of this century, a trend in migration into cities and towns led to an assorted pattern of landscapes, including urban/suburban-dominated areas with forest and agricultural patches, heavily agricultural landscapes, and transitional rural lands with decreasing agriculture and increasing forest regrowth.

Excerpted from:

Thomas C. O'Keefe, Scott R. Elliott and Robert J. Naiman, Introduction to Watershed Ecology, USEPA Watershed Academy Web, Accessed on December 2023, https://cfpub.epa.gov/watertrain/module