A river’s flow regime encompasses the magnitude, duration, frequency, and rate of change in various hydrological conditions. Broad patterns of precipitation and topography determine a river’s natural flow regime, but the natural flow regime may be altered by human activities such as water management (withdrawals, augmentation, storage, and diversions) or widespread land use changes in the watershed. Alterations to natural patterns of flow, including the frequency and timing of floods and droughts, impact fish, aquatic insects, and other biota with life history strategies tied to predictable flow patterns. Changes to peak flows may impact channel stability, riparian vegetation, and floodplain functions. Impacts to base flows may alter water quality and the availability of aquatic habitat. Fluvial ecologists generally treat flow regime as the “master variable” exerting the largest influence on riverine ecosystem form and function.
Sediment Regime
The sediment regime reflects the amount and timing of sediment supplied to a reach from all sources and the corresponding ability of the channel to mobilize and transport that sediment. The sources of sediment to the reach include land erosion in the contributing watershed and channel erosion on reaches upstream. The production and transport of sediment within a stream network is a crucial determinant of channel form and dynamics. Functional characterizations of the sediment regime consider the amount and timing of sediment production, as well as patterns of sediment transport along the stream channel. Evaluation criteria may be based on the number and size of barriers to sediment transport and on the proportion of the watershed from which sediment transport is blocked or elevated above reference conditions. Deforestation, fires or dam construction can all alter the sediment regime Like changes to flow fegime, an altered sediment regime may cause a cascade of impacts to stream form and function.
Water Quality
Water quality includes the chemical and physical characteristics of the water column that influence habitat quality. Water quality in a reach of river is determined by a combination of upstream land and water uses, natural geological weathering, and biogeochemical processing. Water quality parameters are typically the most quantified and monitored aspects of stream health. The existence of regulatory standards and assessment methodologies for a variety of water uses (e.g. aquatic life support, drinking water supply, direct contact recreation, etc.) make assessments of existing or historical water quality conditions relatively straightforward.
Wood Regime
A river’s wood regime, analogous to its flow and sediment regimes, encompasses the processes of wood recruitment (how wood enters the river), transport (how it moves within the river corridor), and storage (where it accumulates). In some settings, the wood regime is a critical aspect of river behavior, influencing channel morphology, habitat diversity, nutrient cycling. In other locations, the role of the wood regime in mediating river health may be more muted due to naturally low rates of recruitment.
Riparian Habitat
The riparian zone is the terrestrial zone adjacent to the river channel that is influenced by the river. The habitats that that exist in the riparian zone are a cornerstone of river health. Riparian habitat is indispensible to wildlife and birds–roughly 80% of Colorado’s species are dependent on riparian areas. Wildlife can move with relative safety through the concealment provided by intact riparian habitat, and birds have ample vertical structure within which to flit and forage. The overhanging canopy offers shade to the river channel, which helps to regulate water temperature. The sub-canopy and understory contribute to habitat diversity for terrestrial and semi-aquatic wildlife. Leaf litter from all vegetation layers serves as a vital food source for aquatic macroinvertebrates. Woody debris sourced from riparian zones is a primary control on a river’s wood regime. Large, woody material delivered to the channel from riparian zones enhances structural complexity of the stream and floodplain, mediating channel hydraulics, sediment distribution, and habitat diversity. Riparian vegetation, in turn, is supported by the river. Floodwaters saturate the riparian soils, scoure floodplain surfaces for new vegetation to take root, and create complex floodplain topography and microhabitats. Persistent baseflows and high water tables sustain rooted vegetation during the hot, dry late summer period. Riparian habitat is most commonly impacted by the direct alterations of floodplain topography (e.g., construction of levees or placement of fill), through land use changes (e.g., agricultural land clearing or urban development), and indirectly by impacts to flow regime.
Channel Dynamics
Interactions between the flow regime, floodplain structure, and riparian vegetation drive dynamic changes to the structure of river channels and streambeds. Channel dynamics encompass processes like lateral channel migration, bar formation, channel avulsion, and riffle crest migration. These dynamic processes, in turn, exert significant influence on aquatic and riparian habitat availability and quality. Lateral channel migration shapes floodplains, creating diverse floodplain topography and complex riparian habitats. Point bar formation is a dominant control on rates of recruitement in many parts of the state. Channel avulsion, while sometimes perceived as a hazard, is a natural process that rapidly disturbs floodplain surfaces. Avulsions can create new wetlands, side channels and sloughs, and redistribute sediment throughout a river corridor. Riffle crest migration helps ensure the persistence of pool-riffle sequences, a critical habitat pattern in many of Colorado’s rivers. The progressive movement of sediment off of riffle crests also helps keep sediments in the riffle “clean”. Constraining these natural dynamic processes through human interventions like levee creation or bank armoring can lead to simplified channel morphology, reduced habitat diversity, and a progressive decline in overall river health and ecological function.
Aquatic Habitat
The variety and spatial arrangement of different physical features within the river channel (i.e., pools, riffles, runs, glides, side channels, sloughs) mediates the availability of habitat niches. Different species and life stages require different hydraulic conditions (water depth, velocity, substrate, cover). Pools offer deep, slow water for resting, refuge from predators, and thermal refuge during temperature extremes. Riffles provide shallow, fast, oxygenated water over coarse substrate, ideal for many aquatic insects and fish spawning. Side channels and sloughs offer low-velocity refuge areas and nursery habitat for juvenile fish. Greater reach-scale complexity generally leads to higher biodiversity and resilience in the aquatic community–more habitat types are available to support diverse needs species and life stage needs. The composition of the streambed directly dictates the suitability of individual channel units (e.g., riffles) for various life stage activities like feeding and spawning. The permeability of the substrate is a critical control on the usability of the streambed by macroinvertebrates an on rates of biogeochemical transformations in the hyporheic zone. Finally, the bi-directional connectivity of habitat at one stream location to habitat at another location in the stream network is critical for fish migration in response to life stage cues or avoidance of stressors. It is also critical for allowing genetic exchange between disparate populations. Many alterations to the channel, land use changes and water mangagement can negatively affect aquatic habitat. Simplification of channel form, bank grading and armoring and removal of large wood all affect coarse-scale habitat. Elevation of sediment supply or loss of flushing flows can change bed composition, clogging interstial spaces in the streambed. Diversion structures, reservoirs, and hydrological dry-up points effectively block movement and migration of aquatic organisms, isolating populations and making them more vulnerable to stressors and pertubations.
Aquatic Food Webs
Aquatic food webs encompass the trophic linkages between primary producers, macroinvertebrates, fish, and other organisms inhabiting aquatic ecosystems. The biotic makeup of a stream is impacted by all other stream health factors. Because aquatic biota depend on all other aspects of stream function, aquatic food webs are often a prime indicator of overall stream health. Ecosystems supporting complex trophic structures and long food chains are more resilient to human disturbance and changing external perturbations like climate change. Activities that impair processes at the watershed, reach, or channel scales often impact food web structure. Impacts to riparian vegetation or disruptions in longitudinal connectivity can limit the supply of leaf-litter to streams, depriving macroinvertebrate communities of an important food source. Elevated nutrient levels in the water column can alter community structures of primary producers and macroinvertabrates, creating a cascading effect through higher tropic levels. Fish stocking of non-native sport fish or introcution of invasive fish, mollusks, and other aquatic biota can have profound impacts on aquatic food webs by increasing inter-species competition, predation, parasitism, and disease.
The Colorado River Health Assessment Framework (CoRHAF) provides a standardized yet adaptable scaffolding for assessing and communicating river health. The CoRHAF is not of itself an assessment method. It is an approach to assessing and communicating river health in a way that meets unique watershed and stakeholder group needs. The CoRHAF reflects a conceptual model that includes fundamental Drivers of river health – things like flow regime, water quality, and riparian vegetation. Drivers broad environmental characteristics or processes that represent whole scientific domains and a exert strong influence on river conditions. The conceptual model that CoRHAF is built upon suggests that evaluation of Drivers and their constituent parts (i.e. Components) will provide a holistic view on river health. The eight default Drivers of river health suggested for use in CoRHAF are noted below:
CoRHAF outlines a process of refining the Drivers with descriptive Components, such as peak flows or water temperature. This process of framework development supports the creation of a river health assessment that is customized to reflect specific local objectives and the availability of data, technical expertise, and funding resources. The evaluation process includes linking sub-optimal river health conditions to expected environmental stressors. Linking degraded Driver condition to stressors in this way can help local groups identify management actions, policies, or projects that can alleviate stressors and improve river health. Generally, Colorado Water Conservation Board (CWCB) expects the CoRHAF to help communities achieve the following goals:
Perform Health Assessments
Successful implementation of the CoRHAF requires evaluating core components of stream health, including: hydrology, geomorphology, water quality, aquatic biology, and riparian ecology at watershed to stream-reach scales. Such a comprehensive approach reduces the likelihood of mis-diagnosis or mis-attribution of degraded stream health.
Engage Local Stakeholders
Assessment efforts that utilize the CoRHAF are well-poised to support productive discussions between community members, resource managers and subject matter experts. These discussion can help a community arrive at a common understanding of river health and articulate a clear and compelling vision for the future condition of local streams and rivers.
Identify Priority Issues and Locations
The CoRHAF is easily adaptable to a variety of strategic planning arenas. Typical outputs are also well suited to stakeholder engagement and community outreach efforts. Reflection on assessment results by community members and subject matter experts can help quickly identify issues and locations that may benefit from focused resource management attention
Standardizing Evaluation Systems
To ensure clarity and consistency, CoRHAF employs an academic grading system for conveying stream health assessment outcomes, regardless of whether supporting information was generated qualitatively through rapid surveys or quantitatively through advanced analyses. The grading rubric may be customized by stakeholder groups and technical teams to reflect unique site circumstances or technical knowledge. Characteristics of river health are assessed using the best available information, which may include past studies and/or new investigations. This approach facilitates clear communication to diverse, potentially non-technical, audiences and helps promote a shared understanding of river condition.
Bridging Science and Public Understanding
The decomposition of aquatic ecosystem characteristics and function into a discrete set of river health Drivers coupled with the use of a straightforward grading rubric help make river health assessment findings translatable and actionable. Practitioners and stakeholder groups can easily translate CoRHAF outputs into a variety of communication media.
TipExample Results Communication Formats
River Health Report Cards
Concise summaries of river health that provide a community with a quick overall impression of local conditions and an opportunity to consider aspects of river health that may warrant protection or restoration.
Data Visualizations
Assessment results may be communicated through development of heat maps, symbolized geographic maps, infographics, causal chain diagrams, and other visual communication tools.
Interactive Community Engagement Tools
Online dashboards, interactive maps, and decision-support systems can provide stakeholders with the opportunity to explore river conditions in a manner that more directly supports a particular community need or planning objective.
CoRHAF provides a structured, adaptable framework for evaluating river health. Through use of the CoRHAF, community members, conservation organizations, resource managers and policymakers—can work together to safeguard the future of Colorado’s waterways.
Supporting Diverse Planning Needs
One of CoRHAF’s defining features is its ability to adapt to the unique needs present in individual watersheds or stream reaches. It serves as both a diagnostic tool and a guide for informed water management decision making. CoRHAF results can inform or serve as a foundation for numerous strategic planning and disaster response efforts, including:
Stream Management Plans (SMPs) – Responding to the Colorado Water Plan’s objectives for support of environmental and recreational water uses.
Watershed Restoration Projects – Identifying degraded areas and prioritizing interventions for improving the condition of streams, rivers, wetlands and riparian areas.
Disaster Recovery Efforts – Assessing and responding to the impacts of wildfires, floods, and other natural disasters.
Policy & Decision-Making Support – Providing data-driven recommendations for water governance at the local, regional, or state level.
