The physical structure of a stream is the result of complex interactions between flow regime, sediment dynamics, and other factors. Patterns of erosion, scour, and deposition shape the streambed by controlling the size and distribution of stream substrates. The more complex and heterogeneous the physical structure of a stream, the more enhanced the habitat for resident aquatic species. Activities that physically alter streambed structure, disrupt the sediment regime, or reduce large wood supplies to a reach often impact the physical structure and degree of heterogeneity in the stream channel.
Assessments of structural complexity consider hydraulic characteristics (water depth and velocity patterns), bed and bank features, and substrate material. These assessments may be divided into coarse-scale assessments (macrohabitats) relevant to fish and larger animals and fine-scale assessments (macrohabitats) applicable to aquatic insects or larval fish. Macrohabitat assessments rate the diversity of water depth/velocity combinations, the shape of bed and bank features, and other large physical structure provided by rock, wood, vegetation, and debris. Microhabitat assessments investigate the degree of interstitial space within the riverbed substrate, degree of embeddedness, bed armoring, and algae in riffles.
Analysis of structural complexity should begin with a review of all available related data. Decisions regarding assessment level and data analysis should be made based on SMP goals and existing data.
Aquatic habitat delineation maps
Inventories of woody material
Maps of large habitat features and bank structures
Wolman pebble counts
Potential Data Sources
Remote assessment of habitat structure (pools, riffles, backwaters)
Qualitative assessment of in-stream and floodplain woody material
Rapid field evaluation of velocity/depth combinations
Mapping and assessment of artificial structures, bank revetments, and constructed channel sections
Streambed sampling (pebble counts)
Interviews with regional aquatic biologists
Tips for Success
Macrohabitat (coarse-scale) and microhabitat (fine-scale) assessments are both critical to different resident aquatic species, and should be weighted equally within the structural complexity variable.
If time or access limit the ability to conduct field visits, aquatic habitat can be evaluated through rapid field assessments in select reaches, and results of both fine- and coarse-scale assessments can then be extrapolated to reaches deemed similar through review of aerial imagery.
Conducting diversion infrastructure assessments can provide additional information about a system’s available aquatic habitat and structural complexity, as well as potential opportunities for addressing in-stream changes that may be a result of in-channel diversions.
Assessing the watershed-scale wood regime can help managers target the root causes of wood deficiency and plan for long-term, sustainable restoration of wood to the river corridor, not just to meet a target amount of wood in the river, but to meet a target function of wood in terms of its geomorphic, hydrologic, and ecologic benefits. Wood regime assessment can also inform the management of flood and geomorphic risks posed by wood.
State of the Poudre: A River Health Assessment
Assessment Level: Level 3
Two metrics were considered to assess the structural complexity condition along the Poudre River though the City of Fort Collins: coarse-scale physical structure (50% of score) and fine-scale physical structure (50% of score). The metrics were graded based on field observations at most project reaches. Grades for the few reaches not observed in the field were extrapolated from similar reaches based on aerial imagery. More information is available in the State of the Poudre report, pages 20-21 and 69-70. This excerpted page from the State of the Poudre Report shows photographs of several examples of coarse-scale and fine-scale habitat conditions.
Crystal River Management Plan
Assessment Level: Level 3
Characterization of stream habitat physical structure for the Crystal River Management Plan aggregated results from prior assessments of flow regime, fluvial geomorphology, debris supply, and aquatic biota to identify locations within the project area expected to exhibit significant departure from natural conditions. Rapid assessments were then conducted in target areas, particularly to observe stress to stream hydraulic structure. Where impairment was identified through field observations and review of other variables, the Ecological Decision Support System (EcoDSS) employed two-dimensional modeling of channel hydraulics and habitat quality and availability to develop habitat suitability curves. These curves assess the degree of departure from natural conditions by comparing the weighted usable area (WUA) of critical fish habitat on each reach under existing conditions (including diversions) and natural conditions (no diversions). For example, for adult rainbow trout, curves show that habitat quality and availability are reduced when diversions are active, but increase during late-season return flows from agricultural fields. Development of these curves can help to assess management impacts (e.g., streamflow alteration) on physical structure.
St. Vrain and Left Hand SMP
Assessment Level: Level 1
To frame their aquatic 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 data collected by CPW and the Left Hand Watershed Center, the group identified key species groups in each watershed zone, including macroinvertebrates, fish, amphibians, and aquatic mammals. They also identified key stressors (e.g., diversion structures that fragment aquatic habitat, 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.
To assess current aquatic habitat conditions, the group evaluated the physical structure of the river for macro-scale features (e.g., pools, water depth, wood, bank features) and micro-scale features (e.g., sediment deposition, algae). They also incorporated observations of biotic structure and organic material into their structural complexity assessments.