A river’s sediment regime consists of the amount and timing of sediment that all sources—including land erosion in the contributing watershed and upstream channel erosion—supply to a reach, as well as patterns of sediment transport along a stream channel. The production, transport, and deposition of sediment largely determines channel form and dynamics. Like changes to flow regime, an altered sediment regime can cause significant impacts to stream form and function, including aquatic habitat quality and long-term channel stability, that an SMP can assess and mitigate.
For example, excessive sediment buildup due to a culvert or other constriction could impair aquatic habitat by embedding the cobbles in the streambed with fine-grained material, limiting suitable locations for aquatic insects and larval fish to occupy. Dams or other blockages to sediment transport can cause result in a phenomenon known as “hungry water,” or river flow with excess transport capacity. In these situations, high flows have more power to transport sediment than available sediment, so they compensate by eroding the streambed and streambanks, compromising channel stability.
Lane, EW. 1955. The Importance of Fluvial Morphology in Hydraulic Engineering. American Society of Civil Engineering, Proceedings 81, Paper 745: 1-17.
Lane’s Balance diagram demonstrates how the channel may respond to changes in various parameters, such as sediment load, channel geometry, slope, and discharge. For example, increasing the sediment load can tip the scale toward aggradation (sediment deposition); to bring the scale back into balance, a change in either the channel geometry, slope, and/or hydrologic load (flow) would be needed.
Evaluation of sediment regime is often 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 a reference condition. The sediment regime can be altered by both natural and anthropogenic causes such as wildfires, dams, deforestation, and urbanization.
Decisions regarding assessment level and the need for sediment source analysis or transport modeling should be made based on SMP goals and existing data.
Bedload and suspended sediment loads
Land and channel sources of sediment
Sediment transport models
Visual indicators of degradation or aggradation
Potential Data Sources
Sediment source, yield, and/or transport analyses or models using available data from: USFS Watershed Erosion Prediction Project (WEPP) model, USGS 3D Elevation Program (3DEP, formerly National Elevation Dataset (NED)), USGS Natural Hydrography Dataset (NHD), USDA Soil Survey Geographic Database (SSURGO)
Remote watershed assessment of sediment sources using current and historical aerial imagery
Reach-specific rapid assessments
Streambed sampling to assess bed composition, embeddedness, and armoring
Interviews with local residents
Tips for Success
Assessments of sediment regime should begin with evaluations of current and historical aerial imagery. Land erosion from the surrounding watershed can be assessed using evidence of land disturbance that is visible on aerial imagery, including road density, devegetated slopes, human-caused mass erosion, and burn scars left by wildfires. Significant instances of localized channel erosion are sometimes visible using aerial imagery as well. In addition, in-line dams that inhibit sediment transport can be identified using aerial imagery.
In areas where sediment transport is a dominant force in shaping stream form, or where evidence of highly dynamic systems is present, conversations with local residents should be pursued as well. These discussions may reveal where sediment loads are perceived to be high, and the causes.
Rapid assessments in the field are a relatively inexpensive way to gain information about streambed substrate composition (grain size distribution); stream power, or the amount of energy that a stream is exerting on its bed, and competency; sedimentation; embeddedness, or the extent to which rocks are sunken into streambed sediment; and armoring, or the size of rocks in the streambed relative to the size of the substrate material immediately below the streambed.
Visual observations of indicators of sediment aggradation and/or degradation are inexpensive, rather simple, and often more reliable than modeling. Examples of visual indicators include bars forming, obvious erosion, and an incised channel with collapsing banks.
In highly dynamic systems where sediment transport is impeded by numerous obstacles, groups can model sediment transport or quantitatively analyze erosion, sediment sources, and sediment yield.
Recent forest fires and associated burn scars can drive sediment regime. Predicting wildfire-prone locations that also may have impacts to human uses (e.g., drinking water supply) before a wildfire occurs may be useful during the SMP process.
Review the most recent to check whether reaches of interest are impaired for sediment. Methodologies for listing certain reaches as impaired based on aquatic life use standards, as well as a narrative description of the sediment parameter, are provided in this document.
Crystal River Management Plan
Assessment Level: Level 3
Early stakeholder conversations in the Crystal River Basin identified sediment dynamics as among the highest concerns for diminished ecosystem function. Many locals observed extremely high sediment loads in the Crystal River, Coal Creek, and other tributaries during late summer precipitation events, and largely attributed the sediment loads to road development and mining in the Coal Basin. Initial assessments consisted of conversations with residents, review of aerial photographs, and rapid field assessments. Because of the system’s dynamism and the public’s concerns about sedimentation, a comprehensive modeling effort was undertaken to quantify relative sediment yields across the watershed. Modeling characterized the impact of geology, topography, vegetation, road development, and other land-use disturbances on hillslope surface erosion, gullying, landslide activity, and road surface erosion (Appendix B of the SMP) to evaluate the sediment regime.
Yampa River Health Assessment and Streamflow Management Plan
Assessment Level: Level 2
To assess the sediment regime within the five study areas through the City of Steamboat Springs, the Yampa River Health Assessment used remote watershed surveys using current and historical aerial photography; review of relevant reports; field visits to significant watershed sites and tributary streams; field surveys, observations during runoff and baseflow conditions, and indicators of excess or depleted sediment supply; and streambed sampling to assess bed composition, embeddedness, and armoring.