Headwaters of the North Fork of the Gunnison River
The fluvial geomorphology variable addresses the processes that interact to control channel form and evolution, including the physical attributes of the watershed (e.g., geology, topography, hydrology), channel hydraulics, sediment transport, and local hillslope and floodplain use practices (e.g., adjacent roadways, grazing). Biological drivers that impact sediment transport (e.g., riparian vegetation, large woody material, beaver activity, aquatic vegetation) subsequently influence channel form as well. Sub-variables include planform (the aerial shape of a river reach), dimension (cross-sectional shape and size), and profile (longitudinal grade or slope).
If a stream reach displays geomorphic patterns that are not consistent with local valley form, hydrology, and sediment regime, it may be more sensitive to disturbance. Human activities that modify the hydrological regime, alter patterns of hillslope erosion, adjust the structure of the channel bed, or modify riparian vegetation may yield fundamental shifts in the geometry and behavior of the stream. Therefore, impacts to fluvial geomorphology often result from the construction of roads and levees on floodplains, streambank armoring, channelization, floodplain encroachment, dam construction, and beaver extirpation.
Decisions regarding assessment level and data analysis should be made based on SMP goals and existing data.
Lateral migration rate
Cross section surveys
Longitudinal profile surveys
Potential Data Sources
Current and historical aerial photographs
LiDAR and other remote sensing imagery
Rapid field assessments by fluvial geomorphologists
Field observation at high and low flow
Conversations with local residents
Tips for Success
Fluvial geomorphology assessments should begin by evaluating current and historical aerial imagery and reviewing available literature. Comparison of current and historical aerial imagery can provide information about stability trends, channel evolution, land use, and direct impacts to channel form (e.g., grade control structures, dams altering sediment transport, floodplain encroachment, armoring, straightening, other evidence of anthropogenic impacts, etc.). Some experts can review aerial photography to remotely measure valley confinement, slope, and planform parameters.
Rapid field assessments by fluvial geomorphologists may measure dimension and profile via cross-section surveys and longitudinal profile surveys. Field visits during high and low flow conditions, along with observations of indicators of flood-prone width and bankfull width, can be used to calculate parameters such as width-to-depth ratio, entrenchment, channel capacity, sinuosity, and channel length/valley length ratio.
Conversations with local residents can be useful for understanding where lateral migration, bank stability, and other channel alterations are a problem. These exchanges can often be an entry for riparian landowners to discussing general river health.
State of the Poudre: A River Health Assessment
Assessment Level: Level 2
Three metrics were considered to assess channel morphology along the Poudre River though the City of Fort Collins: planform, dimension, and profile. The sub-variables were scored using a combination of aerial imagery reviews and field surveys by fluvial geomorphologists with experience on the Poudre River and other Front Range streams. Each reach was scored to reflect the degree of departure from natural reference river form using evidence of anthropogenic impacts, or stressors. More information is available in the State of the Poudre report, pages 19 and 61-66. This excerpted page from the State of the Poudre Report illustrates how historical aerial imagery can be used to glean information about river planform adjustments over time.
Rio Grande, Conejos River, and Saguache Creek Stream Management Plans
Assessment Level: Level 3
A detailed geomorphology assessment is planned for the Rio Grande, Conejos River, and Saguache Creek Stream Management Plans (with some overlap between fluvial geomorphology and sediment regime variable data collection and analysis). The geomorphology assessment is expected to include:
- delineation of geomorphic reach breaks based on geomorphic, hydrological, geological, and infrastructure features;
- utilization of existing GIS layers, aerial photographs, satellite imagery, and field verification to document geomorphic characteristics and constraints (features and stressors) of each reach;
- for priority reaches, analyze bank movement, bar movement, and channel narrowing/widening trends;
- extraction of cross-sections from LiDAR data, pebble count data collection, and Relative Elevation Model (REM) development to support geomorphic trajectory and classification analysis;
- analysis of sediment transport characteristics and channel maintenance flows, and implementation of CSR Stable Channel Design Tool to determine whether sediment transport in and out of the reach trend toward erosion or aggradation; and
- classification of existing geomorphic condition and trajectory of each reach via rapid geomorphic assessments.
Crystal River Management Plan
Assessment Level: Level 2
Evaluation of channel morphology on the Crystal River began with windshield surveys and assessments of aerial imagery to determine the Rosgen stream classification of each reach. Rapid field assessments then investigated states of dynamic equilibrium and identified local impacts on cross-sectional geometry and planform structure.