The RCD measures gravel permeability to determine how suitable streambed gravels are for salmonid egg incubation and larval (alevin) development. This is based on the fact that salmonid egg and alevin survival is correlated with the amount of flow through the gravels in a stream bed. The Napa River is listed as an impaired water body for excessive sediment, which smothers developing fish eggs and larvae buried in the stream bottom. By measuring the rate at which water is able to move through spawning gravels, we can predict survival rates for developing eggs. These data help us determine how sediment deposition in the streambed affects salmonid spawning success.
Stillwater Sciences conducted gravel permeability measurements throughout the Napa River watershed in 2001 as part of the Napa River Watershed Limiting Factors Analysis. RCD partnered with Stillwater to collect additional data in tributary streams and the mainstem Napa River that were not sampled during this initial effort.
- Gravels at potential spawning sites are mixed to a depth of 0.95 feet to simulate mixing and sorting of spawning salmonids during redd construction.
- A steel standpipe with a 1 inch inside diameter and a 2.75 inch-long band of perforations is driven into the substrate so that the band of perforations extended in depth from approximately 0.6 to 0.9 feet below the bed surface.
- Permeability is measured by using a battery-powered vacuum pump to siphon water out of the standpipe to maintain the water level inside the standpipe exactly one-inch lower than the surrounding water.
- By measuring the volume of water siphoned out of the standpipe over a measured time interval, it is possible to determine the recharge rate of the water level in the standpipe.
- At each spawning patch assessed, the standpipe is driven in twice and at least five consecutive permeability measurements are taken.
- Recharge rates (units of volume per time) are converted into permeability (units of length per time) using an empirically derived rating table (Stillwater Sciences).
Gravel Permeability Studies
- Southern Napa River Watershed Plan–2009 (Streams: Browns Valley, Huichica, Milliken, Murphy, Pickle, Redwood, Salvador, Sarco, Spencer, Suscol, Tulucay)
- Central Napa River Watershed Project–2005 (Streams: Bale, Bell, Canon, Conn, Montgomery, Napa River, Rector, Soda, Wing Canyon, York)
- Napa River Fisheries Study–2005 (Napa River Rutherford Reach)