Stream Erosion Condition
Streambank erosion is identified in the Napa River Sediment TMDL as one of the two priority controllable sources of fine sediment to the Napa River stream network, the other being poor condtion of unpaved/dirt roads. Elevated contributions of fine sediment (sand and finer) to the stream essentially clog redds (salmon nests) and increase the frequency of fish egg mortality. Reducing the amount of fine sediment delivered to and accumulating within the streambed can help improve salmonid spawning habitat which is a primary goal of the Napa River Sediment TMDL.
Stream erosion condition is the relative risk of elevated streambank erosion during a subsequent storm event. Stream condition can be assessed in the field using a series of standardized observations and reporting formats using a scale of 0-5. It is assumed that effective stream, riparian and floodpalin enhancement projects will restore critical geomorphic function and improve stream conditions. Stream erosion condition is expressed in ITAS as the percent of high priority stream miles in good condition. The desired target is 70%.
The historic development of the Napa River Valley have cumulatively resulted in riparian encroachment, stream channel straightening and flood control structures to contain flows within enlarged and armored stream channels. The focusing of elevated flows within these constrained channels results in channel adjustments during storm flows including channel widening (i.e. bank erosion) and channel deepening (i.e. incision). These channel adjustments result in over steepened stream banks unable to sustain vegetation and leaving them highly exposed to erosion during future storm flows. Each subsequent winter storm continues to erode susceptible stream banks and bed, exacerbating bank instability and resulting in excessive inputs of sediment directly into the stream. The greater the risk and magnitude of stream bank sediment inputs of sediment directly to the river/stream system.
The annual volume of sediment inputs from streambank and channel erosion can be minimized by the implementation of successful stream, riparian and floodplain restoration efforts and continued riparian maintenance BMPs. Extensive stream restoration efforts have and continue to be undertaken within high priority stream reaches with the goal of improving the stream system's geomorphic function to naturally convey larger storm flows while still providing adequate flood control to adjacent lands. These efforts include riparian setbacks and construction of suitable and accessible floodplains. Changes in channel morphology are implemented to better distribute the energy during storm events and reduce localized shear stress acting on the bed and banks in susceptible locations. Streambank stabilization efforts, such as reducing surface area and/or steepness and planting riparian vegetation, are implemented to reduce erosion rates during storm flows. Functional riparian systems in good condition provide shading, organic material supply, recharge aquifers, improve water quality, support biodiversity, protect the hydrologic cycle and provide an array of other critical ecosystem benefits. Increased miles of priority streams in good condition is expected to increase the cumulative quality and quantity of salmonid rearing, spawning and migration habitat required to sustain and increase the salmonid populations with the Napa River Watershed.
The Streambank Rapid Assessment Methodology (RAM) is a field assessment and information management system developed to evaluate and track the condition of a stream reach on a 0-5 scale. The tool is being cooperatively developed and optimized to directly meet the needs of local agencies, regulators and land owners. Stream RAM functions and features are currently being defined but likely will include:
- A technically defensible approach that leverages existing models and scientific theory to capture sediment transport processes, identify key indicators of impaired vs desired stream attributes, and develop robust scoring algorithms.
- Integrates a series of field observations to define relative stream condition.
- Good conditions may include: functional channel morphology and associated attributes that result in the expected range of channel evolution and erosion during elevated flows, desired riparian vegetation communitites and density, desired substrate complexity and composition, etc.
- Degraded conditions may include: channel morphology and associated attributes that are highly susceptible to erosion, incision and bank failure elevated flows, undesired riparian vegetation communities and density, sand dominated sustrate, etc.
- Employs simple and repeatable stream assessment methods to determine road condition on a relative 0 to 5 scale based on observation results.
- Automates data management and results reporting to evaluate and track the effectiveness of stream improvement and maintenance actions by Napa resource managers and land owners. Stream RAM allows simple exports of annual RAM assessment results in a format directly importable to ITAS as miles of high priority streams in good condition (> 3.5 score).
Napa County stakeholders, including the County, Napa RCD, and private landowners can inventory stream sites in streams that either support, or have the potential to support, anadromous salmonids. Trained field personnel conduct Stream RAM field observations on desired intervals observe and document stream condition. Stream RAM integrates the observation results and reports a RAM score from 0-5 for each reach. These results are automatically integrated by the RAM application to summarize the total miles of high priority stream assessed and the total miles of those high priority reaches in good condition. These results are directly input into ITAS annually by subwatershed.
The target for stream condition is to achieve 70% of high priority road streams are in good codnition year after year. High prioroity streams are defined as those that are known to support anadromy or have a high potential should stream conditions be improved. Good condition is defined as a Stream RAM score >3.5, and the greater the distribution of stream reaches in good condition, the lower the sediment load contribution and higher presumed habitat quality value for salmonids. Tracking of stream condition improvements over time will provide a regional demonstration of the cumulative effectiveness of stream, riparian and floodplain enhancement, restoration and maintenance actions. It is assumed that a significant improvement in the condition of critical stream reaches within the Napa Watershed will directly correspond to a sustained reduction in the amount of annual sediment loading to the local stream network, which in turn is expected to improve salmonid spawning and rearing habitat quality.
The ITAS 70% target is based on what is achievable and assumed ot make a measurable improvement on fish spawning and rearing habitat quality.
