Napa River Nutrient TMDL
The Napa River and its tributaries provide habitat for a diverse array of aquatic life, including steelhead trout and Chinook salmon. Excessive nutrients (nitrogen and phosphorus) have impaired habitat in this river system as well as detracting from the river's recreational value. As a result, the Napa River is on the U.S. EPA's 303(d) list of impaired water bodies due to nuisance algae growth caused by the excessive concentrations of nutrients. Nuisance algae can impair recreational beneficial uses by creating unsightly and unaesthetic conditions.
Algae also blocks sunlight and, when it decomposes, can deplete the river’s supply of dissolved oxygen, which is essential for aquatic life.
Water Board staff is collecting water quality data during summer and fall of 2011 and 2012. We anticipate holding a CEQA scoping meeting in spring 2013 and having a draft TMDL report available for public review and comment in late 2013.
Water Quality Data Collection
During summer and fall in 2011 and 2012, Water Board staff is collecting and analyzing data on physical habitat and water quality at about 10 sites throughout the Napa River watershed. We expect to see wide variation in nutrient levels (nitrogen and phosphorous), the quality of physical habitat (shading, temperature, stream flow) and algae levels (as indicated by the concentration of chlorophyll in samples scraped from stream bed). Because algae can lower night time oxygen levels, we are also continuously monitoring dissolved oxygen at a subset of sites with sampling probes placed in the streams.
We expect to see the highest algae levels where we find high nutrient concentrations and poor quality physical habitat (e.g., low river or creek flow, high water temperature, and low canopy cover). Because algae growth tends to peak in late summer, we will sample sites in the late dry season (August/September), when we would expect to find the worst conditions. These data will help us better understand the scope and severity of the problem in the watershed and will support development of the Napa River Nutrient TMDL.The data collected during the 2011 dry season showed that nutrient concentrations, physical habitat quality, and algae biomass (as given by chlorophyll concentrations) were similar at sites in the Napa River watershed. Overall, few sites showed high chlorophyll levels despite some moderate nutrient concentrations. Most, but not all, sites were below the chlorophyll thresholds thought to be protective of cold water beneficial uses (100 mg chlorophyll per square meter of stream bed). Algae need both nitrogen and phosphorous to grow and, usually, algae growth is limited by one of these two nutrients. Nearly all of the sampling sites had excessive phosphorous, which means that algae growth is limited by the availability of nitrogen. The weather in 2011 was generally cool and wet, so algae growth could have been suppressed by climate. The sampling in 2012 is expected to provide additional insights into the nutrient and algae problem in these watersheds because the data will be collected during a different type of climate year.
For more information contact:
Water Resource Control Engineer
San Francisco Bay Regional Water Quality Control Board
1515 Clay Street, Suite 1400
Oakland, CA 94612