Spawning Gravel Permeability
Salmonid spawning nests, or redds, require coarse sediment to ensure an adequate flow of oxygen to the eggs during incubation. Spawning gravel permeability is the rate of water flow through streambed substrate. High quality spawning gravel permability increases the rate of emergent salmonid survival. Elevated contributions of fine sediment (sand and finer) to the stream essentially clog redds, decreasing spawning gravel permeability and increasing the frequency of egg mortality. Improvement of salmonid spawning habitat quality and quantity is a primary goal of the Napa River Sediment TMDL (Total Maximum Daily Load).
Spawning gravel permeability is expressed in ITAS as the median permeability and standard deviation of the site/reach measurements on a specific day. The desired target is a median permeability of > 7000cm/hr.
The Napa River is the third largest tributary to the San Francisco/San Pablo Bay. Historically, the river supported at least three species of salmonid fish: steelhead (Oncorhynchus mykiss), coho (O. kisutch) and Chinook salmon (O. tshawytscha).Both salmon species were largely extinguished in the twentieth century, but, in the last several decades, Chinook salmon have again been observed in the basin, presumably through a process of recolonization. The successful implementation of sediment TMDL related actions are expected to directly benefit the spawning, rearing and survival of salomids within the Napa River Watershed.
The Napa River Sediment TMDL goals explicitly link the desired reduction of sediment loads delivered to the Napa River streams to the quality and extent of viable salmonid habitat to support rearing, migration and spawning. Elevated chronic sources of sediment to the streams from stream bank and channel erosion, unpaved roads and other sources have impaired the quality and extent of viable salmonid spawning habitat. It is assumed that the implementation of effective actions to reduce sediment sources will increase the distribution and quality of spawning habitat and directly result in increased salmonid populations in the wateshed.
The Napa County Resource Conservation District (RCD) conducts streambed scour monitoring at 8 locations along the mainstem Napa River. The sampling locations have been selected to represent the range of suitable salmonid spawning conditions within the Napa River system. Monitoring targets specific discharge events in the 2-5 year recurrence interval range, which represent a small range of annual peak flow events around the average. Streambed scour is measured within selected reaches by conducting by two methods: (1) repeat topographic surveys before and after a targeted runoff event, and (2) installed scour chains placed across a stream bed to measure the change in the bed surface as a result of the runoff event. The median streambed scour measured at a specific location on a specifc day is reported in units of centimeters. The strandard deviation of the values used to calulate the site median is also reproted. For more information about the RCD's streambed scour monitoring program, contact Jonathan Koehler at jonathan@naparcd.org.
The target for gravel spawning permeability is a site median permeability greater than 7000 cm/hr and that 100% of the sites monitored in any given year achieve that target. The 7000 cm/hr target is based on measured permeability values and scientific literature review as recommended by the Regional Water Quality Control Board (RWQCB). Achievement of this target is estimated to correspond to approximately 50% or greater survival of eggs and larvae from spawning to emergence (RWQCB Limiting Factors Report).
