Physical+habitat

= __Physical Habitat__ = //By Adrienne Martinez and Magdalena Sims//


 * INTRODUCTION **

Alterations to a river or stream’s structure and flow patterns affect the components of a system in many complex and significant ways. The objective of this paper is to look specifically at physical habitat and explore how alterations to a river’s structure and flow affect its instream habitat, aquatic species, and overall health. Defining physical habitat and understanding the tools by which habitat assessment is facilitated will be important in establishing physical habitat as a key component in river restoration planning efforts.

The general attitude towards flows in the Western United States was typically one that encouraged established minimum flows; more recently, however, there has been a shift towards emphasis on the importance of seasonal flows. This paradigm shift has created a need to evaluate how alterations in environmental flows will affect river biota - specifically fish and instream riparian species. The big question here is: how can we effectively measure and analyze the effects of change on physical habitat to instream biota and vegetation?

There are a number of physical habitat assessment methods that have been developed over the last 20 years, some of which focus primarily on physical habitat assessment and others that emphasize geomorphology or instream biota but incorporate physical habitat as a component of assessment (Maddock 1999). Commonly used physical habitat assessment models that will be discussed in this paper include:


 * Physical Habitat Simulation System (PHABSIM)
 * Instream Flow Incremental Methodology (IFIM), which incorporates PHABSIM
 * Ecosystems Function Model (HEC-EFM)
 * Ecopath with Ecosim (EwE)

Although physical habitat assessment can be a useful gauge of river health and a key component in driving river restoration efforts, it is critical to understand its limitations and the different scales upon which assessment methods are based. Physical habitat is an incredibly broad subject, covering both instream characteristics and riparian forest; however, there has been very little research that incorporates riparian habitat into instream physical habitat assessment. A such, this paper will focus specifically on physical habitat within a river.


 * DEFINING PHYSICAL HABITAT **

The overall health of a river is based on a number of physical, ecological, and biological components. River restoration efforts, therefore, must take into consideration each component and its relationship with other elements of the system. Assessing the condition of physical stream habitat is critical in evaluating the overall health of a river (Thomson et al. 2001). To assess the health of a system’s habitat, there must be some understanding of how the system would function in the absence of human influences (Davies et al. 2000).

Physical habitat is spatially and temporally dynamic and is determined by the interactions between structural features and the hydrological regime of a channel (Maddock 1999). Structural features include channel size, channel shape, gradient, and bank structure. Physical habitat also carries with it biological significance (Maddock 1999) because different species thrive in different environments, and these environments are influenced by physical habitat. Clearly, there is a great deal of variability in aquatic habitat from system to system because it is characterized by velocity, depth, cover, temperature, and substrate (Jacobson et al. 2001), all of which are related to a river’s flow regime.

Physical habitat can be evaluated at different spatial scales: macrohabitats, mesohabitats, and microhabitats, all of which will be discussed in greater detail later in this paper. Spatial scales are incredibly important in physical habitat assessment because, for example, restoration efforts that may apply to features at the macro scale may not be beneficial at the micro scale (Maddock 1999). Instream biota such as macroinvertebrates and fish are dependent upon specific meso- and microhabitat characteristics for well-being.


 * WHY IS PHYSICAL HABITAT IMPORTANT? **

According to Stalnaker (1979), water quality, the energy budget (i.e. temperature regime, nutrients, foodwebs ), physical channel structure, and flow regime all contribute to the productivity of a stream system. If the goal of river restoration is to encourage a healthy system, then it is essential that the aquatic habitat within the system sustains and promotes the biotic community.

Habitat assessments serve a number of purposes in river management; namely, they aid in health monitoring, rehabilitation, setting environmental flows, and biodiversity assessment (Thomson et al. 2001). Boon (1992) and Maddock (1999) both point out that the largely negative effects of pollution and river channelization on physical habitat have been well documented, with alterations in water quantity, water quality, and physical channel structure resulting, in most cases, to changes in the composition of the biotic community of the river and often a reduction in the biological diversity.

Most of our rivers have been subjected to anthropogenic influence, resulting in varying degrees of alteration or degradation. Before the health of a river can be properly assessed, it is necessary to characterize the river and understand what features comprise its physical habitat. We are not always able to construct a complete picture of how a river looked and functioned historically in the absence of human activity but we must attempt to do so as accurately as possible in order to tailor restoration efforts appropriately. Considering how physical habitat attributes vary naturally with watershed size, climate and geography can be especially important when assessing physical habitat because changes in any one of these can introduce unwanted stress to the system (U.S. Environmental Protection Agency (EPA) 2010).


 * FLOW REGIME **

Flow regime is defined by the following five key characteristics of streamflow and their associated interactions:


 * Magnitude
 * Timing
 * Frequency
 * Duration
 * Rate of Change

Flow regime is a primary determinant of physical habitat in rivers and, thus, greatly influences biotic composition (Bunn and Arthington 2002). Aquatic species - specifically fish for the purpose of this paper - rely on particular habitat characteristics to propagate and thrive. Petts (2009) points out that flow regime is a complex concept and provides two fundamental general principles related to flow regime: 1) the natural flow regime shapes the evolution of aquatic biota and ecological processes and 2) every river has a characteristic flow regime and an associated biotic community.

Many rivers and streams throughout the world have undergone significant changes in channel shape and structure because of both direct and indirect anthropogenic activities (Maddock 1999) related to flood control, agriculture, and urbanization. Channel modifications have tremendous impacts on instream physical habitat, which is one of the reasons why river restoration efforts should be concerned with environmental flows that mimic a river’s natural flow patterns (Naiman et al. 2002). Although this opinion is held by many in the scientific community, many management approaches fail to acknowledge that the ecological integrity of flowing water systems is highly dependent on their natural dynamic character (Poff 1997). Streamflow, in fact, is correlated with important river characteristics like channel geomorphology, water temperature, and habitat diversity (Poff 1997). It is easy to see the strong link that exists between flow regime, physical habitat, and the health of instream biota (Figure 1).



To further understand just how streamflow affects instream biota, consider the role of flooding. Floods make available critical spawning and nursery habitat to fish (Petts 2009). When the natural flow regime of a river is compromised, so are the fish habitats within and therefore the ecological integrity of the system.


 * ASSESSMENT OF RIVER HEALTH **

The negative impacts of human activity on aquatic ecosystems are becoming all too apparent. These realizations, coupled with the growing awareness of the need to conserve water, are driving forces behind river restoration efforts. Unfortunately, restoration planning is entirely dependent upon the system in question. What may benefit a system on one reach of a river could be entirely different from that which must be done upstream or downstream on the same river.

Some key questions to consider are 1) what defines a healthy system? 2) how does it vary from region to region, from stream to stream? and 3) what are the primary indicators of river health? This is where physical habitat assessment can be utilized as a tool with which to gauge river health and, in fact, it is becoming increasingly important in restoration efforts (Maddock 1999 and Davies 2000). Evaluation of instream habitat is one way to judge the integrity of a river system (Muhar and Jungwirth 1998). A healthy ecosystem is characterized by flourishing native species populations; thus, a river containing healthy biotic composition will likely be indicative of a relatively healthy aquatic habitat.



Physical habitat is not the sole indicator of river health, but it certainly aids in the identification of appropriate conditions for rehabilitation (Thomson et al. 2001). In river health monitoring programs, habitat assessment can be used to gather information about potential causes of instream biological impairment (Thomson et al. 2001). The application of physical habitat assessment carries with it a major assumption, namely that the quantity and quality of available instream physical habitats impact the biological diversity, functionality, and balance of species (Hynes 1970; Meffe and Sheldon 1988; Harper and Everard 1998; Maddock 1999; Thomson et al. 2001).

Having a sense of a river’s health is beneficial for a number of reasons. It allows for more effective river management and also promotes greater understanding of what biological, ecological, and physical components have the greatest effect on overall health of a system. This information is critical to instream habitat evaluation because it can aid in the determination of what levels of detail are worthwhile yet cost-effective (Maddock 1999).


 * RIVER DEGRADATION **

River degradation results from a number of direct and indirect anthropogenic alterations to the natural landscape (Maddock 1999). Channelization, river engineering, hydro-modifications, watershed development, livestock grazing, construction of dams, agriculture, and deforestation are all activities that contribute to the degradation of a river. These activities, while economically beneficial, are detrimental to physical habitat (Jacobson et al. 2001).

There are a number of key considerations when assessing river degradation. For instance, knowing whether biological impairment is a result of habitat degradation or water quality degradation may allow for more tailored and effective restoration efforts (Davies 2000). Another consideration is understanding how exactly activities surrounding the stream affect the aquatic habitat.

An example of the effects of river degradation on aquatic species can be found in New Mexico’s Rio Grande River, in which the Rio Grande Silvery Minnow (RGSM) resides. River degradation here has largely been a result of hydrologic manipulations caused by considerable development of impoundments and diversions (Stone and Sada 2008). The RGSM, an endangered species as listed under the Federal Endangered Species Act (U.S. Fish and Wildlife Service (FWS)) has suffered as a result of habitat degradation caused by hydrologic manipulations. Other factors that have degraded the aquatic habitat of the RGSM stem from stream channelization, loss of floodplain connectivity, invasive species , and watershed and riparian degradation (Stone and Seda 2008). According to Stone and Sada (2008), the RGSM once existed throughout the Rio Grande Basin but today are only seen in the stretch of river that flows from downstream of Cochiti Dam to Elephant Butte Reservoir. Similar cases to that of the silvery minnow can be seen in other river systems, where other types of aquatic species have been affected in a similar manner.

The link between aquatic habitat health and proliferation of a fish species is clear, which is exactly why many scientists see physical habitat assessment as valuable in restoring a degraded system. Maddock (1999) highlights the increased application of river rehabilitation techniques in other countries that include re-creating pools and riffles, reinstating meanders in straightened reaches, artificially narrowing over-wide channels, placing structures on the bed to create morphological diversity, and breaking out concrete channels and culverts.


 * SPATIAL SCALES **

Habitat mapping is a technique of surveying different spatially scaled areas and identifies the physical features of the ecological setting. This type of mapping requires a mixture of qualitative assessment and physical measurements that make up the form and structure of the river. These surveys require manual data collection and, because time is generally limited in the field, the data is not always as complete as would be preferred. The smaller the area to be surveyed, the more detailed and accurate the collected data is likely to be.

Many different habitat defining methods exist today and each method requires different data requirements. These methods are applied over varying spatial scales and are important to define when deciding what restoration measures and fish habitats will be improved. Spatial habitat sizes are broken down into three distinct levels: macro, meso, and micro.

Macrohabitats consist of three levels of stratification - drainage basins, networks, and segments - and are used in modeling analysis. Drainage basins are the largest of the three habitat components and can vary in size, from tens to thousands of square kilometers. Networks are generally made up of two or more sub-basins and segments are considered to be the reaches in the river.

Since large areas and sizes are of concern, macrohabitats are broken down into smaller areas of mesohabitats. Mesohabitats are characterized by a common slope, channel shape, and structure; the length is about the same order of magnitude as the width of the channel. Mesohabitats are commonly known as pools and riffles but can also consist of cascades, glides, and runs. Riffles are an important feature of freshwater channels and have a broken water surface providing excellent hiding areas from predators and, because the current is fast, offer a great source of food. Riffles also oxygenate the water, thus providing ideal congregating locations for fish and other aquatic life. Pools provide depth and still water areas not commonly associated with rivers and offer protection from predators.



Microhabitats define the smallest of scales and are considered to be a combination of the hydraulic and structural features of an area in which aquatic species live. These include the depth, velocity, substrate, and cover available. The range of size for a microhabitat is usually less than one to several square meters. Microhabitats are the most studied areas for specific aquatic species due to the relatively small size areas and accessibility. Clicking on the picture above will take you to the EPA website for Stream Habitat Walk. This approach is a simple way to assess a stream or rivers habitat and relies on visual observation and very little equipment and training. ** INSTREAM FLOW INCREMENTAL METHODOLOGY (IFIM) **  The greatest influences guiding the development of instream flow methods in the late 1960s were dams. Initially, dams were constructed in an effort to improve navigation along rivers and, later, as a way to store water and minimize devastation caused by flood events. It became apparent soon after the implementation of dams across the United States that wildlife, plantlife, and river health were being negatively affected by reduced water flow. The quality and quantity of available habitat for wildlife quickly became a concern for state and federal agencies.  Previously, water managers employed techniques focusing on setting minimum instream flows to define the amount of water in a river below which level fish and other aquatic life could not survive. The late 1970s and early 1980s signified the beginning of small hydropower development in the United States. As federal and state fishery agencies began looking more closely at the hydropower site practices, one of the main concerns focused on the enormous volumes of water released, called hydropeaking, and the effects of these events on the environment. Rapid fluctuations in river flow can cause devastating results in many freshwater biota and the large amounts of water associated with hydropeaking greatly raises the height and flow rate of water in the river. Hydropeaking can also cause fish stranding. When the floodplain becomes inundated with large amounts of water, fish may become stranded on the floodplain as the water recedes back into the river. Aquatic habitats in very shallow depths of water, due to low flow settings, are more susceptible to fatality during a hydropeaking event. Instream Flow Incremental Methodology (IFIM) was developed under the guidance of the U.S. Fish and Wildlife Service and was the answer to handling the problems associated with these types of dam releases. Working with dam releases, water managers intend to mimic the historical seasonal flows of the river using incremental methods and are moving away from the idea of using minimum flows.  "IFIM is based on the analysis of habitat for stream-dwelling organisms under alternative management treatments. One could logically question why habitat was chosen as the decision variable in IFIM when there are so many other factors (such as stream productivity or fishing mortality) that can potentially influence fish populations. The simplest reason for basing the analysis on habitat is that IFIM was designed to quantify environmental impacts, and impacts to habitat are the most direct and quantifiable." (Stalnaker et al. 1995; [|USGS])  In 1970, the National Environmental Policy Act ([|NEPA]) was enacted. This new act encouraged the decision to change water management practices from setting fixed minimum flows to using incremental methods. Early research focused on life-stage-specific relations for selected species (fish passage, spawning, and rearing habitat versus flow). From these studies came the development of habitat versus discharge functions. Researchers correlated the health of fish populations to the physical and chemical attributes of the flow regime and found that the following set of variables greatly affected the changes in fish survival:
 * Water velocity
 * Minimal water depths
 * Instream objects such as cover
 * Bottom substrate materials
 * Water temperature
 * Dissolved oxygen
 * Total alkalinity
 * Turbidity
 * Light penetration through the water column

These variables became the basis of incremental methods and are used to determine how water is managed in the withdrawal and storage-release activities of rivers and federal water projects. Water quality and quantity for ‘potential habitat’ became the foundation for the IFIM and once again took into account the relationship of habitat versus flow. IFIM is a simple and general problem solving approach that uses five successive phases for planning: problem identification, study planning, study implementation, alternative analysis, and problem resolution. ** PHYSICAL HABITAT SIMULATION SYSTEM (PHABSIM) **  PHABSIM is a computer modeling system that is part of the IFIM and, like IFIM, is based on flow versus life-stage-specific habitat requirements. PHABSIM uses field measurements of channel shape, velocity, depth, and substrate of a microhabitat along with combined hydraulic modeling and habitat knowledge of specific aquatic species. The result is a simulation of the quality and quantity of the existing habitat versus flow relationships based on current conditions and a ‘potential habitat’ using smarter proposed water development. PHABSIM is able to calculate the amount of microhabitat available for different life stages at different flow levels of a target species. ** PHYSICAL HABITAT SIMULATION ON THE RIO GRANDE RIVER **  There are numerous methods of assessing instream physical habitat being applied in the U.S. that are based on habitat versus flow modeling, similar to IFIM and PHABSIM. Stone and Sada (2007) demonstrated how they developed a habitat evaluation model that was able to predict habitat suitability for the Rio Grande Silvery Minnow (Hybognathus amarus) depending on varying stream flows in the Rio Grande River in New Mexico. The length of river that was used in the study was a 3 km reach in the Middle Rio Grande from the Alameda Boulevard Bridge to the Paseo Del Norte Bridge through the Albuquerque area. Using a 2D horizontal stream flow and sediment transport modeling system called CCHE2D (developed by the National Center for Computational Hydroscience and Engineering at the University of Mississippi) they evaluated habitats within the river for the Rio Grande Silvery Minnow (RGSM). Under the Federal Endangered Species Act, the states of New Mexico and Texas as well as the Republic of Mexico listed the RGSM as an endangered species. The Rio Grande Basin was once teeming with RGSM; currently, however, the fish can only be found in a 280 km reach of the river downstream of the Cochiti Dam and above the Elephant Butte Reservoir. The silvery minnow is a pelagic-broadcast spawner and releases thousands of eggs during the spring runoff or the early summer monsoons. The eggs are semi-buoyant and drift with the current for up to 50 hours while the hatched larvae can drift for up to an additional day. The larvae are weak swimmers and must find low-velocity habitats to survive. The increasing degradation of RGSM habitat is directly related to the effects from channelization, damming, and other hydrologic and physical manipulations to the river. Due to river modifications and increased river velocities, the RGSM eggs and larvae now drift farther than they did historically. This is merely one example of how a species (i.e. the RGSM) life-stage and population size can be affected by alterations to physical habitat.

 Stone and Sada applied channel topography data obtained by a HEC-RAS model to the CCHE2D model and imported those results into an ArcGIS module. After applying the habitat module, the data were then imported into a computer software program called FRAGSTATS, which was used to evaluate the quantity and quality of available habitat over varying ranges of streamflows. Both steady and unsteady flows were simulated in this study. For steady flow, it was found that the most desirable habitat suitability occurred at 700 cfs, which is below bankfull discharge (~2,500 cfs), due to the low flow velocities. Regardless of size, the RGSM most often occupy mesohabitats with low water velocities, moderate depths, and small substrata. As the model increased flows to overbanking rates, the amount of desirable habitat on the floodplains also increased and contained areas of low velocities.  In the unsteady flow simulation, streamflows were raised from 200 cfs to 10,000 cfs and then back to 200 cfs. Using this simulation, the problem of stranding (where a fish becomes trapped in an inundated floodplain as water recedes back into the river) was studied and the issue of habitat disconnectivity was discovered. The information collected from this study will be used to improve water management and restoration of the Middle Rio Grande and help to improve habitats and populations of the silvery minnow. ** HYDROLOGIC ENGINEERING CENTER-ECOSYSTEM FUNCTIONS MODEL (HEC-EFM) **  The Ecosystem Functions Model (EFM) models the ecosystem responses to changes in the flow regime of a river. HEC-EFM uses statistical analyses of relationships between hydrology and ecology, hydraulic modeling and the use of Geographic Information Systems (GIS). HEC-EFM results show existing ecologic conditions and can map out areas of desirable restoration sites and can assess and rank alternatives for predicted changes within an ecosystem. The modeling system is not dependent on a single, specific relationship to test ecological changes, but rather multiple relationships and flow regimes. ** ECOPATH **  Another computer modeling simulation software system being used around the world is Ecopath with Ecosim (EwE). This type of modeling looks at habitat availability according to the food web system. EwE combines the Ecopath software for ecosystem trophic mass balances (biomass and flow) analysis and the dynamic modeling Ecosim software to investigate the past and future impacts of alterations to habitat. The Ecosim models can be combined with Ecospace to view policies over a spatially dynamic map grid of protected areas. Ecopath has a wide variety of uses and addresses many ecological issues. It can assist in evaluating the effects of fishing on an ecosystem, in analyzing the impact and placement of marine protected areas, and in modeling the effect of environmental changes. EwE also employs software that can predict the flow and accumulation of contaminants and tracers in Ecotracer.



** BIOENERGETICS ** PHABSIM has been criticized because it relies heavily on empirically based curves showing the relation between flow and habitat and is limited in its approach. PHABSIM works well when the following three criteria are met:
 * Appropriate life stages of the target species have been studied and their habitat preferences have been defined
 * The bathymetric (water topography), hydraulic, and hydrologic characteristics of the habitat have been accurately measured and/or simulated
 * Physical habitat quantity and/or quality is the limiting issue

Many scientists, biologists, engineers, and researchers believe that bioenergetics may be a better way to link habitat models and biological mechanisms (which IFIM and PHABSIM lack). An energetic approach provides information linked to growth and biomass production and is connected closely to food web dynamics. Using prey capture models a potential net energy intake rate (NEI) is calculated of a given stream position by subtraction energy costs and losses from the gross energy intake rate (GEI) that is found by simulating prey capture. The main emphasis of this type of modeling is to find the optimum feeding positions (habitat) of a particular fish species by modeling the energy it takes for a fish to maintain its position while feeding. As mentioned previously, it also takes into account the food web. ** LIMITATIONS OF PHYSICAL HABITAT ASSESSMENT ** River restoration planning is not a black and white effort and, unfortunately, is not just a matter of selecting a single system component and ‘fixing’ it. There are a number of defining characteristics that determine river health. Physical habitat is one such component. It has the potential to be a strong tool in river restoration planning, but there are a number of significant drawbacks that should be addressed. One of the primary issues with employing physical habitat assessment methods is the fact that they are not as well developed as other methods that are used to evaluate water quality, water quantity, and biotic integrity - all key aspects of river health (Maddock 1999). Many existing habitat assessment methods are uniscalar and do not relate habitat to physical processes (Maddock 1999; Davies et al. 2000). This is obviously a highly limiting factor because different scales are relevant to different biota and, therefore, complete assessment of habitat condition must be multi-scalar (Thomson et al. 2001). Physical habitat assessment can be further limited by the inability of restoration planners to properly identify accurate reference conditions (Maddock 1999; Davies et al. 2000; Thomson et al. 2001). Without an adequate understanding of a river's historical condition and functionality, it can be difficult to evaluate current habitat health and quantify results of physical habitat assessment. Along the same lines, invalid assumptions about what constitutes "good physical condition" can be made if habitat attributes of one system are erroneously applied to another system that may, in fact, be similar in a broad sense but still differ too greatly on smaller scales to be lumped into the same grouping (Thomson et al. 2001). ** CURRENT APPROACHES IN THE WESTERN UNITED STATES AND POLICIES RELATED TO PHYSICAL HABITAT ** The importance of river health, restoration, and quality has only recently become better recognized in the western United States. Unfortunately, not all of the western states are truly comprehending the urgency of acting now. While California, Washington, and Oregon have been trailblazing ahead, implementing new methods and policies for assessing and protecting physical habitat, other states like New Mexico and Arizona have fallen far behind. California’s Environmental Protection Agency website, [|waterboards.ca.gov], boasts a wealth of information pertaining to plans, policies, laws, and regulations for the health of their rivers and streams. California’s policy for maintaining instream flows in the state's northern coastal streams focuses on protecting native fish populations. The policy is intended to focus particularly on anadromous salmonids like the steelhead trout (Oncorhynchus mykiss), coho salmon (Oncorhynchus kisutch), and chinook salmon (Oncorhynchus tshawytscha), and their respective habitats. These three species were placed on the “threatened” list under the federal Endangered Species Act (ESA) and the California Endangered Species Act (CESA) in 1996. The coho salmon’s status was upgraded from threatened to endangered on both list in 2005. According to the Department of Fish and Game in 2004, degradation and loss of freshwater habitat is one of the leading causes for the decline of salmonids in California. Under this policy principles and guidelines have been developed for maintaining instream flows that protect fishery resources. The policy was designed to protect the river from having to flow at minimum levels during seasonal and other types of diversion. It also limits the construction of new onstream dams. The new dams that are approved are monitored to ensure that they do not negatively affect the instream flows necessary for the river and the fishery resources. Colorado has also made an effort to protect its rivers and streams by implementing the Colorado River Cooperative Agreement. Signed on April 28, 2011, the agreement promises to protect river habitat among many other water issues including responsible water management throughout the Colorado River Basin. This agreement is intended to bring together opposing parties related to water management and usage and focus on cooperation to change the way water is managed in Colorado. The goal is to improve the environmental health of the Colorado River Basin while still maintaining the recreational use of the river and meeting the municipal and economic needs of many cities, counties, and businesses impacted by the Colorado River. The flowing waters of the Colorado River are governed by multiple interstate and international compacts making it one of the most legally complex rivers in the world. The laws and policies affecting the river are numerous and can be accessed through the [|Western Water Assessment] website. New Mexico and Arizona do not have explicit policies set in place for the future protection of the rivers and streams within their respective states. New Mexico's [|Fish and Fisheries Laws] currently have "no provisions that relate explicitly to fish kills or habitat destruction due to nonpoint source pollution." Judging solely on the fact that some states are trying very hard to implement plans and policies to protect and improve their instream habitats and others aren't, there is obviously a huge failure throughout the United States. There needs to be communication between the western states to share information regarding what works, what does not, and how to bring people together to agree and make decisions for healthier systems and smarter water management. ** CONCLUSION ** Instream physical habitat assessment is arguably an important component of river health assessment and restoration planning. Some of the research that has been done in the past twenty to thirty years highlights the necessity of modeling aquatic habitat on various scales in order to gauge the effects of past and present anthropogenic activity on instream biota. The procreation, well-being, and sustainability of fish and other aquatic species are highly dependent on 'healthy' biological, physical, and ecological components within a system. Research clearly demonstrates the importance of considering attributes of physical habitat in overall assessment of a river's current condition. The limitations of physical habitat assessment, however, are significant. Assessment methods, for example, do not incorporate key system features like water quality and riparian habitat into their overall analysis. Additionally, there is no established baseline by which characteristics of different rivers may be compared because of the significant variability in habitat from system to system. Not only is there high variability from system to system, but there are notable differences in characteristics along reaches of a single river. The issue of evaluating physical habitat on multiple scales only further confounds the issue of standardizing assessment techniques and approaches.  Another key consideration in physical habitat assessment is that of data collection. The various modeling systems employed in habitat assessment require different data inputs. Some modeling systems are far simpler than others, requiring generalized physical habitat data. More complicated models demand more extensive data, which means greater manpower, time, and money must be allocated to collecting the required data. The heavy financial commitment that can be associated with physical habitat assessment causes some to wonder if it is truly worthwhile. Research would suggest that it is, in fact, a worthwhile investment but that there is a lot of room for research into how physical habitat assessment methods can be standardized. ** REFERENCES ** Boon, P.J. (1992). Essential elements in the case for river conservation. 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