Spawning Gravel Permeability

Spawning Chinook SalmonSalmonid 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 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).