Well-functioning riparian corridors contain complex assemblages of plant species with specific structure, diversity, and processes that interact directly with the river. Riparian areas provide many critical functions for maintaining healthy and resilient stream ecosystems. These functions include supplying physical roughness that slows water velocities and mitigates the impacts of flood flows; increasing bank stability through root system cohesiveness; hosting a diversity of riparian plants, animals, and microbes; filtering surface runoff that may carry elevated sediment from forest fires, logging, or urban development; providing shade to lower water temperatures for improved aquatic habitat; contributing large wood to stream channels (thereby providing habitat, in-stream cover, pool creation, and sediment trapping); adding organic matter; and creating off-channel habitats like backwaters, wetlands, and side channels that act as refugia for aquatic species. Well-established and connected riparian areas also link stream corridor and upland ecological processes.
Riparian vegetation can be directly and indirectly compromised. Direct impairments result when a riparian area is converted to developed urban or agricultural land. Development and land conversion are often inevitable, and necessary for many human needs. Riparian buffers, or areas between a stream channel and developed area that remain intact, can be helpful for maintaining ecological integrity, filtering nutrients that may result from urbanization and agricultural development, and protecting agricultural lands.
Indirect impacts can stem from alterations to flow and sediment regime, ecological connectivity, and floodplain hydrology. This variable rates the degree to which the supporting aspects of vegetation structure and composition, connectivity, and ecological processes are impaired by human impacts such as land conversion and land management.
Decisions regarding assessment level and data analysis should be made based on SMP goals and existing data.
Riparian corridor extent
Floodplain position or connection to the river
Age class structure
Vertical complexity or number of strata
Rare and significant species
Presence of invasive species
Potential Data Sources
Review of aerial imagery (with ground truthing), including LiDAR data where available
Reach-specific rapid assessments
Field transect sampling using various methodologies
Interviews with County weed control agencies and review of other noxious weed resources
Conversations with local residents
Tips for Success
Assessments of riparian vegetation should begin by evaluating current and historical aerial imagery and review of available literature. With limited ground-truthing, reviews of aerial imagery are sufficient to evaluate vegetation structure on urban, developed, bare ground, and open water land-cover types. Patches of simple herbaceous vegetation such as turf and lawns are also easily evaluated remotely, but field assessments may be needed to suitably evaluate sites with more complex natural vegetation.
In most cases, rapid field assessments by vegetation ecologists are employed to evaluate complex vegetation patches in areas of interest and characterize vegetation conditions across the study area. Field assessments should consider the dominant valley form (e.g., confined versus unconfined) and local floodplain size when assessing the degree of departure from reference conditions.
For SMPs with specific targeted questions related to vegetation condition, modeling efforts such as the recruitment box model may be applied. The recruitment box methodology compares rates of water table decline to average root growth rates for woody riparian vegetation to identify optimal and suboptimal conditions for sapling growth.
Crystal River Management Plan
Assessment Level: Level 3
Characterization of vegetation condition on the Crystal River relied mainly on literature reviews and rapid field assessments. In areas affected by hydrological impairment, the Ecological Decision Support System (EcoDSS), developed to understand historical and current flow regime characteristics along the Crystal River, implemented the recruitment box methodology to provide a quantitative understanding of constraints related to cottonwood recruitment success. Simulation of one-dimensional channel hydraulics at representative cross sections replicated rates of hydrograph recession and falling water surface elevations. Cottonwood saplings require a limited rate of water table decline to ensure that growing roots maintain contact with the water surface. By comparing the number of days exhibiting optimal recruitment conditions at natural versus existing conditions, researchers were able to assess the degree of departure from reference conditions.
State of the Poudre: A River Health Assessment
Assessment Level: Level 3
Three metrics were considered to assess the vegetation condition along the Poudre River though the City of Fort Collins: vegetation structure (80 percent of score), habitat connectivity (10 percent of score), and contributing area (10 percent of score). For the vegetation structure metric, the riparian zone was mapped remotely by delineating patches (or polygons) and classifying them according to land cover. For complex patches that could not be classified solely by aerial imagery, nine sub-metrics were scored in the field: vertical complexity, canopy and sub-canopy species, shrub layer, problem herbaceous and woody species, patchiness, native woody species regeneration, and floodplain position. Degree of departure from reference condition was scored for each reach to calculate a “report card” score for the riparian vegetation condition. More information is available in the State of the Poudre report, pages 17-19 and 55-61.
Functional Assessment of the Mancos River Watershed
Assessment Level: Level 3
The Mancos River Resiliency Group used a Functional Assessment report completed in 2007 for baseline data regarding the functional condition and ecological health of the Mancos River Watershed. The basin-wide assessment was conducted on private lands throughout the watershed and used protocols outlined in the User’s Guide for the Rapid Assessment of the Functional Condition of Stream-Riparian Ecosystems in the American Southwest (Stacey, et al. 2006).
The evaluation of riparian condition divides the riparian area into lower and upper riparian zones. For each zone within each study reach, estimates of percent cover for grasses, forbs, shrubs, and middle and upper tree canopy are made, and then summarized in an overall mean score by reach. Additional variables examined and factored into the overall reach score include
- demographic structure and recruitment of the dominant native shrub and tree species along each reach;
- estimated relative amount of cover provided by non-native species versus native species;
- level of grazing by mammals on grasses and forbs; and
- level of browsing by mammals on shrubs and trees.
St. Vrain and Left Hand SMP
Assessment Level: Level 3
To frame their terrestrial habitat conditions assessment in Phase 1 Report Section 5.3, the St. Vrain and Left Hand SMP identified conservation targets (species, biological communities, or ecosystems that are the focus of habitat planning) and compiled data on stressors known to increase habitat vulnerability. Using various mapping and data collection techniques, the group identified target plant, macroinvertebrate, wildlife, and fish species in each watershed zone. They also identified key stressors (e.g., wildland fire potential, lack of proper flow conditions, climate change, invasive species, increasing recreational use, water quality impacts from mining) in each watershed zone to consider in planning. Wildland fire potential (high and very high forest fire risk in the watershed) was mapped using the LANDFIRE dataset.
Specific to identifying the potential to reduce riparian habitat stressors, the plan assessed riparian plant communities, instream habitat features, and floodplain connectivity. Indicators of wetland and riparian habitat health included overall size and extent; native plant cover, diversity, structure, and evidence of regeneration; and extent of non-native species. The plan used the following techniques to evaluate riparian conditions:
- use of a combination of aerial analyses to map and classify riparian land cover and extent within the corridor;
- quantification of riparian land cover within the creek corridor buffer, defined as ¼ mile on either side of the main channel;
- intersection analysis of hydrogeomorphology and wetland occurrences in the corridor;
- observations of condition based on riparian vegetation width, incidence of non-native crack willow, and age classes of native cottonwoods ranging from seedlings to decadent.