Selecting Drivers, Components, & Metrics
There is no fixed set of CoRHAF Drivers, Components, and Metrics that must be evaluated by a stakeholder group or technical team. Instead, CoRHAF provides opportunities for each river health assessment to make use of a unique set, while providing some structured guidance for selecting Drivers, Components, and Metrics that are responsive to the unique needs of a project. This section describes how the CoRHAF structure can be adapted to the unique circumstances of your assessment.
The Drivers of river health form the core of CoRHAF. In most situations, all Drivers must be evaluated in order to provide a comprehensive assessment of river health. The default list of Drivers will be sufficient to capture the breadth of stream and river processes and characteristics that contribute to ecosystem condition in most cases. However, circumstances may arise that lead technical teams and/or stakeholder groups to conclude that the default list of Drivers requires supplementation or revision. For example, the Wood Regime may not be a relevant consideration for a health assessment focused on a high alpine streams. In other locations, the winter ice regime may be as critical as flow and sediment regimes in driving channel morphology and dictating aquatic habitat quality. Where Drivers are added or removed, the groups or individuals making those changes should provide documented rationale for their actions.
Most groups will find the greatest opportunity for customization of CoRHAF at the Component and Metric levels. Each Driver can be decomposed into innumerable Components. In turn, each Component can be characterized by way of innumerable Metrics. Groups that choose to revise the default CoRHAF Components list should adhere to the notion that Components are conceptual representations of some facet of form or function of a Driver. Components may be assessed qualitatively by technical experts but they are not directly measurable, by their nature. For example, a hydrologist may be able to provide a general impression of Peak Flow conditions on a river segment after a review of stream gauging records and consideration of the number and size of upstream reservoirs. However, the concept of Peak Flows cannot be quantified on its own. Metrics comprise the first quantifiable level in the CoRHAF hierarchy. A useful Metric of Peak Flow behavior might be the average annual 3-day maximum flow, but it is not the only Metric capable of characterizing Peak Flows.
Framework customization impacts the development of grading guidelines and the level of effort required to assess conditions. Some efforts may elect to assess river health at the Driver or Component level. In these cases, a review of quantitative data may be included as a step in the assessment process but all grading guidelines and assigned river heath scores will be, by their nature, qualitative. Qualitative assessments may be supported by desktop exercises (e.g., review of aerial imagery), rapid field assessments and other approaches that correspond to a relatively low resource burden. Other efforts may seek specific, quantifiable reflections on the condition and function of one or more Components. In these cases, more resource-intensive, quantitative evaluations will be required. These quantitative assessments may require field data collection, simulation modeling, and application of other methods that are difficult to implement if time or funding resources are limited.
No effort is made here to provide an exhaustive list of the potential Drivers, Components or Metrics that may be included for any single assessment effort. Instead, assessment teams must choose the best CoRHAF structure for the unique characteristics of the river and the particular social and resource circumstances that impact a project. Deciding which Drivers, Components, and Metrics to include is a deliberative process that requires reflection on the following:
- Purpose of the assessment
- Unique stakeholder interests
- Geographic scale of the assessment area
- Availability of financial and technical resources
- Availability and quality of existing information and data
With these issues in mind, the technical team can select the Components and Metrics that are most-responsive to the local river context and the needs and desires of stakeholders. Further discussion on each of the bullet items above is provided in the sections below.
Reflecting on Assessment Purpose
Stakeholder groups and technical teams have the opportunity (and the obligation) to consider the purpose and motivations for conducting a river health assessment as they seek to select a final list of Drivers, Components and Metrics. The ultimate goal of a river health assessment is to answer questions about river condition and function. However, these questions may be diverse. In some cases, stakeholder groups may seek a better understanding of baseline conditions across a watershed. Yet in other cases, groups may seek information that can support policy development or implementation of management practices and projects that protect or enhance river health in response to some known or expected stress to the system. A myriad other possibilities exist. Four examples are discussed below.
Scenario 1
Watershed groups and other stakeholder groups may undertake river health assessments to gain a clear understanding of the current ecological conditions on stream reaches throughout a local watershed. Without a broad understanding of a river health, resource restoration and protection efforts risk being misdirected or ineffective. In order to identify locations or issues with the greatest need for protection or restoration, assuming no prior knowledge about existing stressors, finite funding resources, and typical project completion timelines, such efforts may benefit from a framework customization that delves no deeper than Drivers and Components. This structure accommodates qualitative grading guidelines and expert-based assessments of condition that can be rapidly applied across large geographies. Generating qualitative baseline information on river health can be crucial to watershed groups’ ability to effectively identify problematic conditions, track those conditions over time, and ultimately develop targeted strategies for river protection and/or restoration. Results from such an effort can also be used to build public outreach and community engagement efforts around identified sources of river health degradation.
Scenario 2
Results from broad-scale qualitative assessments of river health may motivate a more detailed assessment at a particular location in a watershed where degradation of one or more river health Drivers or Components was observed. In these cases, it may not be necessary to build out an assessment framework that include Metrics for all Drivers and Components. Rather, it may be sufficient to select a structure that includes quantitative Metrics responsive to a subset of key Drivers and Components while retaining a coarser structure for other Drivers and Components. This approach can help answer important questions about the nature of observed impairments–how acute they are, whether they appear to act synergistically with other degraded conditions, etc.–without requiring a comprehensive and resource intensive assessment.
Scenario 3
River health assessments can be used to motivate and track outcomes of restoration projects. These applications likely require detailed assessment of the Components of river health that the restoration project aims to improve. This scenario is similar to Scenario 2, but focuses more intensive investigation on a subset of improved, rather than degraded, Drivers and Components. Consider an example restoration project that aims to improve the condition of Riparian Vegetation and Connectivity by planting willows and modifying floodplain topography. River health assessments conducted pre- and post-restoration and utilize an assessment framework that includes Metrics for tracking Aquatic Connectivity (e.g., inundation extent of the 5-year flood) and composition of the Shrub Stratum (e.g., percent woody cover). This approach can be useful for tracking restoration successes or failures and placing those outcomes in context of broader river functioning.
Scenario 4
Assessments completed in response to an observed or expected change in local river conditions may benefit from detailed framework structures that support intensive analytical investigations of one or more Components. For example, if the objective of a health assessment is to characterize the impacts of a recent or proposed reservoir development project in order to affect permitting processes and/or the development of reservoir operating rules, the preferred CoRHAF structure may include a high number of Metrics across the various Components (e.g., Peak Flow, Base Flow, Sediment Supply, Physical Water Quality, Aquatic Connectivity, Macroinvertebrate Community Structure) impacted by altered water management. Such a structure would necessitate resource-intensive, quantitative evaluations but would yield results more suited to supporting a policy position or developing a rationale behind a particular set of reservoir management practices.
Targeting Highly Influential Factors
Drivers are foundational building blocks of river health. They interact and produce a river’s characteristic conditions and functions. All rivers are influenced to some degree by the default list of Drivers included in the CoRHAF, but some Drivers may have either an outsized or reduced role in supporting river health in different settings. For example, the importance of the riparian zone in maintaining river health is vastly different between tightly confined and unconfined rivers. A selected assessment framework structure for a canyon-dominated river like the Black Canyon of the Gunnison, might treat Riparian Vegetation in a qualitative manner at the Driver or Component level. In an unconfined alluvial system like the Yampa River near Hayden, the riparian zone is likely important enough in Driving river health to warrant inclusion of one or more quantitative Metrics. In other settings, water quality standard exceedances stemming from historical mine drainage may dominate the health of the river. In these cases, assessment framework structures that elevate water quality by including numerous Metrics for dissolved metals may be appropriate. As a general rule of thumb, more important Drivers of river health should be treated more intensively than other Drivers. Determination of which Drivers are more or less important in a given setting is a deliberative process that should engage stakeholders and subject matter experts.
Accomodating Specific Stakeholder Interests
The particular interests of one or more individuals or organizations can motivate selection of particular assessment framework structures. Stakeholder interests may be congruent with concepts of river health, may run parallel to those concepts, or may run counter to them. Components and Metrics used in river health assessment should relate directly to river health in unbiased and objective ways. Particular interest in avian species (e.g., Yellow-billed Cuckoo) that inhabit riparian areas may motivate extension of the Aquatic Food Webs Driver to include a Component responsive to birds. Regardless of specific stakeholder interests, the selected assessment framework should relate directly to river health and not the ecosystem services delivered by healthy rivers.
Customized CoRHAF structures should only include Drivers, Components, and Metrics that are important to river health, not the community or individual benefits that arise from a healthy river. For example, a stakeholder group may be interested in angler success and may advocate for its inclusion in the assessment framework. However, angler success is not a component of river health, it is a valued outcome of a river that is healthy enough to support a population large adult fish. Angler may also be influenced by competition among anglers, non-preferential flow conditions for wade fishing, fish stocking activities, and other, non-river health related conditions. In this way, angler success may be a product of river health but it does not influence river health outcomes. A technical team trying to be responsive to community values around angling can instead select Components and Metrics that evaluate important aspects of of fish habitat quality or aquatic food web dynamics (e.g., baseflow, dissolved metals, channel hydraulics, channel bedforms, macroinvertebrate population diversity, etc.). The findings of an assessment strutured in this way can help stakeholders identify the river health characteristics and processes that may degrade or support a robust fishery.
Grappling with Geographic Scale
River health assessments may seek to characterize conditions across a variety of spatial scales, spanning a range from the channel scale (tens to hundreds of feet) to the reach scale (hundreds of yards) and the watershed scale (tens to hundreds of square miles). The scale of the assessment will strongly influence the customized assessment framework structure in most cases.
A limited number of quantitative assessment methods can be easily scaled to cover an entire watershed. Exceptions may exist in streamflow and water quality data. Hydrological simulation modeling results or stream gauge records can provide relatively detailed characterizations of Flow Regime throughout a stream network. Water quality data generated at a specific location are generally indicative of conditions up and downstream to a major tributary, landscape stressor or change in geology. For most other Drivers, an assessment framework structure for a watershed-scale assessment will stop at the Driver or Component level. This approach accepts qualitative evaluation methods that can be rapidly employed across large land areas.
Assessments of river health focused on much smaller sections of stream are better suited to the application of intensive quantitative methods. They are also much more likely to be guided by motivating questions that target a particular condition, stressor, or community concern. At these smaller scales, development of assessment framework structures that include a greater number of Metrics for evaluating Components may be warranted.
Cases intermediate to the two above also exist. In a watershed scale assessment, it is possible that particular stream reaches carry particular weight in driving conditions across large areas (e.g. a reach affected by acid mine drainage). Alternatively, one tributary may exhibit some characteristic that is of particular interest to the local community (e.g. a native cutthroat trout fishery). In either setting, it is reasonable to expect an assessment framework structure that is detailed down to the Metric level in areas of particular interest and limited to the Driver or Component level elsewhere.
Contemplating Resource Availability
Carrying out a rigorous health assessment is not a trivial task in terms of time and resources required. Customization of the CoRHAF structure allows stakeholders and technical teams to tailor the assessment approach to available resources. As the desired level of objectivity and/or defensibility of conclusions increases, so too does the intensity of the required assessment activities. In settings where a high degree of objectivity is desired, an assessment framework structure that includes a large number of Metrics for characterizing Components is preferred. However, this type of assessment can only be carried out where sufficient funding resources and technical expertise is available.
Including or excluding Metrics for quantitative evaluation of Components is akin to a control knob that can be dialed in to influence the cost and technical resources required for carrying out an assessment. Qualitative assessments are less expensive and quicker to implement. Thus, a river health assessment effort with limited access to funding or technical expertise will probably realize the greatest successes where the customized CoRHAF structure is built out to the Components level but includes few, if any, Metrics.
The relationship between assessment level intensity and the defensibility of results can vary. Under certain circumstances, a low intensity approach can still yield convincing evidence. For instance, simple desktop evaluation of a burn scar in aerial imagery may yield usable information regarding impaired sediment regimes or vegetation conditions.
Working Within Time Constraints
Most health assessments are carried out in one or two field seasons and the methods utilized must yield meaningful information within that amount of time. The Components and Metrics selected to describe and evaluate Drivers in an assessment are closely coupled to methods available for evaluation. While many options are possible, choices must reflect reasonable expectations for assessment implementation timelines. There is little point in selecting Metrics that are highly relevant to a Component but cannot be assessed within the expected project completion timeline. Some Metrics may require long-term data sets. For example, evaluation of various characteristics of the flow regime requires many years of streamflow data from a given location. The lack of available observed or simulated streamflow data may, thus, preclude inclusion of flow regime Metrics in an assessment framework. Other metrics may require long-term observation of slow or episodic processes. For example, direct measurement of bank erosion rates may help describe Corridor Dynamics and Sediment Regime. However, collecting sufficient amounts of data to characterize a bank erosion rate Metric likely requires a minimum of 3-5 seasons for field data collection (and up to 10 seasons in some cases). It is probably not feasible for an assessment effort operating on a 1 or 2 year timeline to rely on this type of information type for assessment purposes.
Evaluating Existing Information
The amount and form of existing data and information available to a river health assessment can strongly influence the composition of a customized assessment framework structure. Where quantitative data or published research findings relevant to some Component exists, Metrics can be selected to make use of that data or information. Where data and information gaps exist for a Driver or Component, the decision to build out the CoRHAF structure to the Component vs. Metric level should reflect considerations of resource availability, geographic scale, time constraints, etc.
