The complexity of jurisdictions and sheer number of individuals involved in the management of wildland fire requires the establishment of a common lexicon of terms. For those unfamiliar with fire terminology we offer links to a widely accepted glossary of terms and provide connections to training materials highlighting fire fundamentals.
Fire, if properly controlled and managed, can be a valuable tool to manipulate vegetation composition, structure, and fuel loads on rangelands (and other wildland ecosystems). Managed fire can create and maintain a mosaic of plant communities, at appropriate locations on the landscape, to provide valuable ecological services that benefit many rangeland resources. Among these are:
The use of fire as a management tool requires that it be applied at the correct frequency (return interval), intensity (heat release), and spatial scale so that burned and unburned patches remain on the landscape. A mosaic of burned and unburned patches typically results in a mix of early- to late-seral communities, which benefits more rangeland resources than a homogeneous landscape of either early- or late-seral plant communities.
The Importance of Fire Intervals
Prior to European settlement of North America, most American rangelands burned periodically. The return interval between fires varied widely among different rangeland ecosystems (e.g., tall-grass prairie, sagebrush steppe, ponderosa pine forest) but was relatively constant within an ecosystem. Some pre-settlement fires were caused by lightning, but others were intentionally set by Native Americans to achieve vegetation management objectives. Because fire, regardless of the source, was a recurring ecological event with a relatively constant interval, most rangeland vegetation has adapted to periodic burning. After a fire, the vegetation usually forms early-seral plant communities. If there are few unburned "islands," wildlife species that need mid- to late-seral plant communities to complete their annual life cycle cannot inhabit the area or they have very small populations. The same ecological effect occurs when fire return intervals become so short that mid- and late-seral plant communities never become established. Excessively long or short fire return intervals create uniform plant communities across large landscapes and tend to exclude (or dramatically reduce) the wildlife species that need a diverse habitat structure created by a mosaic of early- to late-seral plant communities.
Modern fire return intervals on most rangelands are either longer or shorter than the pattern that evolved prior to settlement. This has resulted in undesired vegetation changes that often adversely affect rangeland resources. Fire return intervals that are substantially longer than the evolved interval lead to a substantial increase in shrubs and/or trees (woody fuel) and a decline in many desired grasses and forbs. The long life span of most shrubs and trees, combined with the slow decomposition (compared to herbaceous vegetation) of their dead branches, stems, and leaves creates large fuel loads and a fuel ladder that can carry a fire into the upper canopy of the vegetation. Horizontal and vertical continuity of the vegetation (fuel), combined with excessive fuel loads, can result in large, intense wildfires across tens of thousands of acres or more. In the western United States, it is becoming increasingly common for individual fires to burn 100,000 to 200,000 acres or more.
Public land management agencies and some private owners of range and forestland are increasing their effort to thin the woody vegetation on land they administer or manage. Their goal is to reduce wildfire risks and create plant communities that are more resilient to insects, disease, and the inevitable wildfire that will occur. Treatments that thin shrub- and tree-dominated communities not only reduce fuel loads but can also provide large volumes of woody biomass for an alternative fuel for society. According to a recent national study, about 8.4 billion tons of dry biomass resides on range and forest lands, but only 60 million dry tons can be removed annually with current fuel reduction methodologies (USDOE and USDA, 2005). At the current removal rate, it will take at least 140 years to reduce woody fuel loads; thus, additional large-scale catastrophic fires are inevitable in many areas.
"On these landscapes four options exist for fire management... Simply put: You can do nothing, and leave the fires to God and nature. You can try to exclude fires and suppress those that do break out. You can do the burning yourself. Or you can change the combustibility of the landscape such that fires, whether from accident, arson, lightning, or prescription behave in ways you favor...Proper fire management requires bits of each, mixed to proportions suitable to the taste of particular sites."
(Tending Fire: Coping with America's Wildland Fires by Stephen J. Pyne, 2004, pg. 69).
Science can help us understand historic fire regimes, describe the benefits and impacts to ecosystems from different fire scenarios, and model fire behavior. Only humans can decide their role in fire management. Resource and property damage compel suppression, but at what cost? Some fires probably should be started or allowed to burn. How do we decide which ones? Thinning overgrown brush and forests can help prevent catastrophic crown fire. Can this be done in an ecologically sensitive manner that we all agree upon? How widely should it be applied? What role does wildfire play in global climate change and how do we decide when open fire is threatening public health? Decisions about how to respond to fire will be political ones based on society's values.
As appreciation of the important ecological role played by fire has increased, so has the debate over how to respond to wildfire and when to use prescribed fire as a management tool. Some fear that fire has become a panacea, and risks being applied indiscriminately. Others point to the successful achievement of management goals when prescriptions are right. For certain, more people are living on and adjacent to highly flammable range and forest lands creating formidable challenges for fire management. Some fires given current fuel loads are beyond our control. How will we respond to fire? Here are links to basic fire concepts, information about fire management, and highlights of some of the burning issues related to fire on Western Rangelands.
Rangelands across the globe are tightly coupled with local and regional climates, benefitting from periods of enhanced precipitation and suffering during prolonged drought periods. Climate variability and change pose unique challenges to livestock producers, pastoralists and land managers the world over. An increased understanding of large-scale modes of climate variability like the El Niño-Southern Oscillation have improved seasonal forecasts and can aid in rangeland drought planning and preparedness efforts, helping to sustain production operations and guide land stewardship. A changing climate shifting towards warmer conditions and increasing hydroclimatic variability is further raising the stakes on becoming ‘climate-smart’ in using climate monitoring and forecast information in managing rangelands.
Alfisols are moderately leached soils that have relatively high native fertility. These soils have formed primarily under forest and have a subsurface horizon in which clays have accumulated. Alfisols are found mostly in temperate humid and subhumid regions of the world.
Alfisols occupy ~10.1% of the global ice-free land area. In the United States, they account for ~13.9% of the land area. Alfisols support about 17% of the world's population.
The combination of generally favorable climate and high native fertility allows Alfisols to be productive soils for both agricultural and silvicultural use.
Alfisols are divided into five suborders: Aqualfs, Cryalfs, Udalfs, Ustalfs, and Xeralfs.
Adapted from: The Twelve Soil Orders: Alfisols. University of Idaho, College of Agriculture and Life Sciences.
Andisols are soils that have formed in volcanic ash or other volcanic ejecta. They differ from those of other soil orders in that they typically are dominated by glass and short-range-order colloidal weathering products such as allophane, imogolite, and ferrihydrite (minerals). As a result, andisols have andic properties - unique chemical and physical properties that include high water-holding capacity and the ability to "fix" (and make unavailable to plants) large quantities of phosphorus.
Globally, Andisols are the least extensive soil order and account for only about 1% of the ice-free land area. They occupy about 1.7% of the U.S. land area, including some productive forests in the Pacific Northwest region.
Andisols are divided into eight suborders: Aquands, Gelands, Cryands, Torrands, Xerands, Vitrands, Ustands, and Udands.
Adapted from: The Twelve Soil Orders: Andisols. University of Idaho, College of Agriculture and Life Sciences.
Aridisols are calcium carbonate-containing soils of arid regions that exhibit at least some subsurface horizon development. They are characterized by being dry most of the year and having limited leaching. Aridisols contain subsurface horizons in which clays, calcium carbonate, silica, salts, and/or gypsum have accumulated. Materials such as soluble salts, gypsum, and calcium carbonate tend to be leached from soils of moister climates.
Aridisols occupy about 12% of the Earth's ice-free land area and about 8.3% of the United States.
Aridisols are used mainly for range, wildlife, and recreation. Because of the dry climate in which they are found, they are not used for agricultural production unless irrigation water is available.
Aridisols are divided into seven suborders: Cryids, Salids, Durids, Gypsids, Argids, Calcids, and Cambids.
Adapted from: The Twelve Soil Orders: Aridisols. University of Idaho, College of Agriculture and Life Sciences.
Entisols are soils of recent origin. The central concept is that these soils developed in unconsolidated parent material with usually no genetic horizons except an A horizon. All soils that do not fit into one of the other 11 orders are entisols. Thus, they are characterized by great diversity, both in environmental setting and land use.
Many entisols are found in steep, rocky settings. However, entisols of large river valleys and associated shore deposits provide cropland and habitat for millions of people worldwide.
Globally, entisols are the most extensive of the soil orders, occupying about 18% of the Earth's ice-free land area. In the United States, entisols occupy about 12.3% of the land area.
Entisols are divided into six suborders: Wassents, Aquents, Arents, Psamments, Fluvents, and Orthents.
Adapted from: The Twelve Soil Orders: Entisols. University of Idaho, College of Agriculture and Life Sciences.
Gelisols are soils of very cold climates that contain permafrost within 2 meters of the surface. These soils are limited geographically to the high-latitude polar regions and localized areas at high mountain elevations. Because of the extreme environment in which they are found, Gelisols support only about 0.4% of the world's population — the lowest percentage of any of the soil orders.
Gelisols are estimated to occupy about 9.1% of the Earth's ice-free land area and about 8.7% of the United States. Although some Gelisols may occur on very old land surfaces, they show relatively little morphological development. Low soil temperatures cause soil-forming processes such as decomposition of organic materials to proceed very slowly. As a result, most Gelisols store large quantities of organic carbon; only soils of wetland ecosystems contain more organic matter. Gelisols of the dry valleys of Antarctica are an exception; they occur in a desert environment with no plants and consequently contain very low quantities of organic carbon.
The frozen condition of Gelisol landscapes makes them sensitive to human activities.
Gelisols are divided into three suborders: Histels, Turbels, and Orthels.
Adapted from: The Twelve Soil Orders: Gelisols.University of Idaho, College of Agriculture and Life Sciences.
Histosols are soils that are composed mainly of organic materials. They contain at least 20 to 30% organic matter by weight and are more than 40 cm thick. Bulk densities are quite low, often less than 0.3 grams per cubic centimeter.
Most Histosols form in settings such as wetlands where restricted drainage inhibits the decomposition of plant and animal remains, allowing these organic materials to accumulate over time. As a result, Histosols are ecologically important because of the large quantities of carbon they contain. These soils occupy about 1.2% of the ice-free land area globally and about 1.6% of the United States.
Histosols are often referred to as peats and mucks and have physical properties that restrict their use for engineering purposes. These include low weight-bearing capacity and subsidence when drained. They are mined for fuel and horticultural products.
Histosols are divided into five suborders: Folists, Wassists, Fibrists, Saprists, and Hemists.
Adapted from: The Twelve Soil Orders: Histosols.University of Idaho, College of Agriculture and Life Sciences.
Inceptisols are soils that exhibit minimal horizon development. They are more developed than Entisols but still lack the features that are characteristic of other soil orders.
Inceptisols are widely distributed and occur under a wide range of ecological settings. They are often found on fairly steep slopes, young geomorphic surfaces, and resistant parent materials. Land use varies considerably with Inceptisols. A sizable percentage of Inceptisols are found in mountainous areas and are used for forestry, recreation, and watershed.
Inceptisols occupy an estimated 15% of the global ice-free land area. Only the Entisols are more extensive. In the United States, they occupy about 9.7% of the land area. Inceptisols support about 20% of the world's population — the largest percentage of any of the soil orders.
Inceptisols are divided into seven suborders: Aquepts, Anthrepts, Gelepts, Cryepts, Ustepts, Xerepts, and Udepts.
Adapted from: The Twelve Soil Orders: Inceptisols.University of Idaho, College of Agriculture and Life Sciences.
Mollisols are the soils of grassland ecosystems. They are characterized by a thick, dark surface horizon. This fertile surface horizon, known as a mollic epipedon, results from the long-term addition of organic materials derived from plant roots.
Mollisols primarily occur in the middle latitudes and are extensive in prairie regions such as the Great Plains of the United States. Globally, they occupy about 7.0% of the ice-free land area. In the United States, they are the most extensive soil order, accounting for about 21.5% of the land area.
Mollisols are among some of the most important and productive agricultural soils in the world and are extensively used for this purpose.
Mollisols are divided into eight suborders: Albolls, Aquolls, Rendolls, Gelolls, Cryolls, Xerolls, Ustolls, and Udolls.
Adapted from: The Twelve Soil Orders: Mollisols.University of Idaho, College of Agriculture and Life Sciences.
Oxisols are very highly weathered soils that are found primarily in the intertropical regions of the world. These soils contain few weatherable minerals and are often rich in iron (Fe) and aluminum (Al) oxide minerals.
Oxisols occupy about 7.5% of the global ice-free land area. In the United States, they only occupy about 0.02% of the land area and are restricted to Hawaii.
Most of these soils are characterized by extremely low native fertility, resulting from very low nutrient reserves, high phosphorus retention by oxide minerals, and low cation exchange capacity (CEC). Most nutrients in Oxisol ecosystems are contained in the standing vegetation and decomposing plant material. Despite low fertility, Oxisols can be quite productive with inputs of lime and fertilizers.
Oxisols are divided into five suborders: Aquox, Torrox, Ustox, Perox, and Udox.
Adapted from: The Twelve Soil Orders: Oxisols.University of Idaho, College of Agriculture and Life Sciences.
Spodosols are acid soils characterized by a subsurface accumulation of humus that is complexed with Al and Fe. These photogenic soils typically form in coarse-textured parent material and have a light-colored E horizon overlying a reddish-brown spodic horizon. The process that forms these horizons is known as podzolization.
Spodosols often occur under coniferous forest in cool, moist climates. Globally, they occupy ~4% of the ice-free land area. In the US, they occupy ~3.5% of the land area.
Many Spodosols support forest. Because they are naturally infertile, Spodosols require additions of lime in order to be productive agriculturally.
Spodosols are divided into 5 suborders: Aquods, Gelods, Cryods, Humods, and Orthods.
Adapted from: The Twelve Soil Orders: Spodosols. University of Idaho, College of Agriculture and Life Sciences.
Ultisols are strongly leached and acidic forest soils with relatively low fertility. They are found primarily in humid temperate and tropical areas of the world, typically on older, stable landscapes. Intense weathering of primary minerals has occurred, and much calcium (Ca), magnesium (Mg), and potassium (K) has been leached from these soils. Ultisols have a subsurface horizon in which clays have accumulated, often with strong yellowish or reddish colors resulting from the presence of iron (Fe) oxides. The red clay soils of the southeastern United States are examples of Ultisols.
Ultisols occupy about 8.1% of the global ice-free land area and support 18% of the world's population. They are the dominant soils of much of the southeastern United States and occupy about 9.2% of the total U.S. land area.
Because of the favorable climate regimes in which they are typically found, Ultisols often support productive forests. The high acidity and relatively low quantities of plant-available Ca, Mg, and K associated with most Ultisols make them poorly suited for continuous agriculture without the use of fertilizer and lime. With these inputs, however, Ultisols can be very productive.
Ultisols are divided into five suborders: Aquults, Humults, Udults, Ustults, and Xerults.
Adapted from: The Twelve Soil Orders: Ultisols.University of Idaho, College of Agriculture and Life Sciences.
Vertisols are clay-rich soils that shrink and swell with changes in moisture content. During dry periods, the soil volume shrinks and deep wide cracks form. The soil volume then expands as it wets up. This shrink/swell action creates serious engineering problems and generally prevents formation of distinct, well-developed horizons in these soils.
Globally, Vertisols occupy about 2.4% of the ice-free land area. In the United States, they occupy about 2.0% of the land area and occur primarily in Texas.
Vertisols are divided into six suborders: Aquerts, Cryerts, Xererts, Torrerts, Usterts, and Uderts.
Adapted from: The Twelve Soil Orders: Vertisols. University of Idaho, College of Agriculture and Life Sciences.
The National Cooperative Soil Survey identifies and maps over 20,000 different kinds of soil in the United States. Most soils are given a name, which generally comes from the locale where the soil was first mapped. Named soils are referred to as soil series. Soil survey reports include the soil survey maps and the names and descriptions of the soils in a report area. These soil survey reports are published by the National Cooperative Soil Survey and are available to everyone.
Soils are named and classified on the basis of physical and chemical properties in their horizons (layers). “Soil Taxonomy” uses color, texture, structure, and other properties of the top two meters of soil to key the soil into a classification system to help people use soil information. This system also provides a common language for scientists.
Soils and their horizons differ from one another, depending on how and when they formed. Soil scientists use five soil factors to explain how soils form and to help them predict where different soils may occur. The scientists also allow for additions and removal of soil material and for activities and changes within the soil that continue each day.
Factors Contributing to Soil Formation
Parent material - Few soils weather directly from the underlying rocks. These residual soils have the same general chemistry as the original rocks. More commonly, soils form in materials that have moved in from elsewhere. Materials may have moved many miles or only a few feet. Windblown loess is common in the Midwest. It buries glacial till in many areas. Glacial till is material ground up and moved by a glacier. The material in which soils form is called “parent material.” In the lower part of the soils, these materials may be relatively unchanged from when they were deposited by moving water, ice, or wind
Sediments along rivers have different textures, depending on whether the stream moves quickly or slowly. Fast-moving water leaves gravel, rocks, and sand. Slow-moving water and lakes leave fine textured material like clay and silt when sediments in the water settle out.
Climate - Soils vary, depending on the climate. Temperature and moisture amounts cause different patterns of weathering and leaching. Wind redistributes sand and other particles, especially in arid regions. The amount, intensity, timing, and kind of precipitation influence soil formation. Seasonal and daily changes in temperature affect moisture effectiveness, biological activity, rates of chemical reactions, and kinds of vegetation.
Topography - Slope and aspect affect the moisture and temperature of soil. Steep slopes facing the sun are warmer. Steep soils may be eroded and lose their topsoil as they form. Thus, they may be thinner than the more nearly level soils that receive deposits from areas upslope. Deeper, darker colored soils may be expected on the bottom land.
Biological factors - Plants, animals, microorganisms, and humans affect soil formation. Animals and microorganisms mix soils and form burrows and pores. Plant roots open channels in the soils. Different types of roots have different effects on soils. Grass roots are fibrous near the soil surface and easily decompose, adding organic matter. Taproots open pathways through deeper layers. Microorganisms affect chemical exchanges between roots and soil. Humans can mix the soil so extensively that the soil material is again considered parent material.
The native vegetation depends on climate, topography, and biological factors, plus many soil factors such as soil density, depth, chemistry, temperature, and moisture. Leaves from plants fall to the surface and decompose on the soil. Organisms decompose these leaves and mix them with the upper part of the soil. Trees and shrubs have large roots that may grow to considerable depths.
Time - Time is also a component for the other factors to interact with the soil. Over time, soils exhibit features that reflect the other forming factors. Soil formation processes are continuous. Recently deposited material, such as the deposition from a flood, exhibits no features from soil development activities. The previous soil surface and underlying horizons become buried. The time clock resets for these soils. Terraces above the active floodplain, while similar to the floodplain, are older land surfaces and exhibit more development features.
These soil-forming factors continue to affect soils even on stable landscapes. Materials are deposited on their surface and blown or washed away from the surface. Additions, removals, and alterations are slow or rapid, depending on climate, landscape position, and biological activity.
Soil is the basic component of rangeland ecosystems and is associated with nearly all processes that occur within the ecosystem. It provides a medium to support plant growth. It is also the home for many insects and microorganisms. It is a product of parent material, climate, biological factors, topography, and time. The soil formation process is slow, especially in arid and semiarid climates. It is believed to take several hundred years to replace an inch of top soil lost by erosion. Rangeland soils, as those found in the Great Plains and Palouse Prairie, have been extensively converted to agricultural crop production. Remaining rangeland soils may be rocky, steep, salt affected, or otherwise not very productive compared to prime agricultural lands.
Any decision made by a rangeland manager must consider the impacts of that decision on the soil. It is important to know the type of soil present on rangeland and to understand how it affects both the kind and amount of forage produced and the type of management that is possible or appropriate. The chemical and physical characteristics of a soil determine: its ability to furnish plant nutrients, the rate and depth of water penetration, and the amount of water the soil can hold and its availability to plants.
Erosion/Deposition Components of Riparian Health:
While the primary function of riparian channels is to convey water and sediment, they also dissipate stream energies. Energies can be dissipated within the channel through boulders or large wood, or by sinuosity. If flows and energies are high enough, then floodplains help dissipate the energies, especially in wide valleys. Floodplains act as overflow buffers and serve a critical function in mitigating the downstream impacts of floods. Floodplains comprise the area adjacent to channels over which out-of-bank flows are diffused. These areas reduce the energy by spreading out the high flows. If floodplains are vegetated with riparian plants, then the vegetation will further absorb energies and slow the flows.
Different Channel Types Mean Different Ways to Dissipate Energy. Channel characteristics vary by channel type which must be identified in order to evaluate streams correctly, and understand the ways in which energies are dissipated. For example, a B2 channel type is moderately entrenched, typically located in or on coarse alluvium, and has a limited floodplain. This channel type has channel characteristics of boulders and small cobbles as well as some floodplain for expansion and often woody riparian vegetation to dissipate stream energy. On the other hand, a C4 channel type is a slightly entrenched and gravel-dominated stream, which has to have access to a floodplain and channel characteristics such as backwater areas, oxbows, and overflow channels along with riparian vegetation to dissipate stream energy.
Channels which have lost those characteristics that help them to dissipate energy are quickly degraded.
Warning Signs. Some floodplain and channel characteristics warning signs to look for that may indicate declining health or “unraveling” of riparian areas include:
The Formation and Maintenance of Point Bars. The formation and extension of point bars into channels is a natural depositional process for some channel types. Point bars are formed when the higher velocity flows on the outside of meander bends erode the bank, and then deposit that sediment on the inside of a meander bed where velocities are lower. It is crucial that vegetation colonizes these deposits as they extend over time to maintain a balance. Point bars forming and then gaining finer sediments stabilized by vegetation is how folldplains form in these channel types. If vegetation cannot maintain a balance, energies during high flows accelerate erosion, which affects sinuosity, gradient, and access to the floodplain resulting in the degradation of the riparian-wetland area. The vegetation present needs to be riparian-wetland plants that have root masses capable of withstanding high-flow events. Point bars are most common in Rosgen's B and C channel types.
Warning Signs. Some warning signs that sediment is not being captured on point bars, which may indicate declining health or “unraveling” of riparian areas include:
What is Lateral Stream Movement? Streams located within non-confining landforms are constantly in the process of moving back and forth across the valley floor. This lateral stream movement is a natural process and enables the stream to slow its velocity. It has been defined as meandering or snaking through the landscape, patterned after how a snake moves over land. When this movement is excessive, it can have serious impact on the overall function of a riparian wetland area, limiting its ability to dissipate energies.
Lateral Stream Movement is a Natural Process. Lateral movement of stream channels is a natural process in wide gently sloping valleys. Because sinuosity and lateral stream movement are a function of landscape setting, lateral movement is strongly related to how the stream maintains balance with its landform. Lateral movement occurs through bank erosion, and its rate is influenced by many factors, especially stream type, nature of bank material, and kinds and amounts of vegetation on the streambank, as well as how point bars are functioning within the system.
Bank erosion must be evaluated relative to stream type, particularly as a stream type manifests itself in the bank material sizes present at a site. For example, a meandering riffle-pool stream channel likely will exhibit higher bank erosion rates in areas of sandy material than in areas where silts and clays provide some cohesiveness to the bank. Thus, “natural” rates of channel migration will vary by stream type and material. Streams or rivers with very stable banks and well rooted vegetation become deep and narrow, highly sinuous, and they flood often. If streams lose bank stability and become wider as they move faster, their length shortens as they become straighter. This can also happen if sediment supply is increased. Some streams naturally occur in areas with sandy or gravelly sediment or higher sediment loads and are like this naturally. When meandering streams lose too much bank vegetation and stability or receive too much sediment, they can become braided. Braided channels move rapidly in each high flow. Their multiple channels split apart and come back together. In some settings, a braided channel is natural and in others it represents unnatural conditions.
Warning Signs. Some warning signs that indicate that sediment is not being captured on point bars, and that the system may be declining or “unraveling” include:
Some channel types are limited to lateral movement by existing landforms such as bedrock, and should be considered laterally stable.
Channels Downcut over Time. Stream channels transport water, sediment, and other materials out of the watershed, thus reducing the overall elevation of the landscape, including the valley bottom. This channel lowering, although part of the natural cycle of landscape evolution, usually occurs at rates that are detectable only over very long periods of time (i.e., hundreds of years or more). Occasionally, natural disturbances or human activities are significant enough to produce rapid vertical adjustments of a channel or channel network that are measurable (several feet or more) in relatively short periods of time (decades or less). These rapid vertical adjustments are known as downcuts (or incisement) or headcuts.
Rapid Downcutting is an Indicator of a Problem. Downcutting of a channel is an indication a riparian system may not be functioning properly. After natural or human disturbance to a system, downcutting occurs when this disturbance causes the stream to increase its velocity and erode away the channel bottom. An incised channel is one in which the average flood--of the size and intensity which occurs roughly every one to three years--is unable to "access its floodplain" by overflowing its streambanks. As the channel cuts downward, the groundwater table is lowered. Consequently, water-loving plants are isolated on the old floodplain and streambanks, and may no longer get the moisture they need.
Determine the Cause. If vertical instability of the stream is suspected, it may be useful to determine if downcuts are the result of local conditions such as a breaching of a beaver damn on site, or if there is something larger happening within the watershed that might be causing the instability. It is important to note that it is often difficult to differentiate between local and watershed-wide processes without extending the investigation upstream and downstream of the site in question.
The Process of Restabilizing. During basinwide adjustments, there are a series of stages of channel evolution that the channel will undergo. For example, a well-vegetated, healthy stream which frequently uses its floodplain often will undergo bed-level lowering following a perturbation (such as a breach of a beaver dam). This downcutting usually results from excess stream power in the disturbed reach. Bed-level lowering eventually leads to oversteepening of the banks, and when critical bank heights are exceeded, banks collapse into the channel, causing mass wasting, which leads to channel widening. As mass wasting and channel widening proceed upstream, an aggradation phase follows in which a new low-flow channel begins to form in the sediment deposits at the new lower level. Upper banks may continue to be unstable at this time. The final stage of evolution is the development of a channel within the deposited alluvium with dimensions and capacity similar to the predisturbance channel. The new channel is usually lower than the predisturbance channel, and the old floodplain now functions primarily as a terrace. Where vertical channel adjustments are systemwide, this sequence of evolution usually manifests itself longitudinally along the stream profile and often along the tributaries as well in the form of headcuts moving up through the stream system.
Warning Signs. Some warning signs in regards to vertical stability to look for that may be indicative of declining health or “unraveling” of riparian areas include:
An Example of Recovery. A common situation in the West is shown in this figure. Here past events created an incised channel and changed the relationship of the stream to its surrounding landscape. The water can no longer get near the top of the old streambanks. As a result, vegetation on top of these old streambanks has become an upland community. However, the stream apparently has ceased downcutting, and a new riparian area is developing in the bottom of the incised channel. The stream can reach this new floodplain. The vegetation established along the new streambanks and on the new floodplain is performing the functions of a riparian community.
The Balance Between the Channel and the Water and Sediment. Viewed over very long periods of time and under relatively stable climate regimes, undisturbed channels and their floodplains exist in a state of relative equilibrium. Streams transport water and sediment out of a watershed, and channels are constantly adjusting to account for changes in the water and sediment. These erosion and deposition processes generally offset each other. Excessive erosion or deposition indicates that this process is out of balance. In order to determine if the stream is in balance, the following questions must be answered
Are sinuosity, width/depth ratio, and gradient in balance with the channel? Could the uplands be contributing to riparian degradation? Are floodplains and channel characteristics able to dissipate energy? Is the channel vertically stable?
Warning Signs. Some warning signs that might indicate that a stream is out of balance with the water and sediment load, that may indicate declining health or “unraveling” of riparian areas include: