Measuring Flow in California's Sacramento–San Joaquin Delta: the USGS Flow-Station Network

A modern flow station, with a sideward-looking acoustic Doppler current profiler on a piling. USGS.

The U.S. Geological Survey installed the first gage measuring flow from the Sacramento River into California's Sacramento–San Joaquin River Delta in the late 1800s. Today 35 hydroacoustic stations measure flow across the delta, the hub of California's freshwater supply. Sampled every 15 minutes and posted online every hour, their data guide daily decisions on how much water can be pumped, where and when — and help protect fish harmed by pumping and habitat loss in what has been called the "most managed and highly altered" watershed in the country.

USGS-operated sites in the Sacramento–San Joaquin River Delta. USGS.

The challenge: tides

• Huge tides: peak tidal flows of 1.7 million cubic feet per second move through the Golden Gate. Twice a day tides push into the delta, reversing flow as far upstream as Freeport, less than 15 miles below Sacramento.
• Fast water: currents can top 2 miles per hour; a parcel of water at Liberty Island at high tide can be past the Rio Vista Bridge, 8–9 miles down-estuary, 6 hours later.
• Signal and noise: at Jersey Point, tidal flows reach about 150,000 cfs while the net (freshwater) flow may be 2,000 cfs or less, so tidal measurements must be accurate to better than 1 percent — or the net flow could even come out in the wrong direction.

How it grew

Technology

Until the 1970s, a single snapshot of net flow took a 30-hour campaign with several boats, current meters and tag lines stretched across channels. Now sideward-looking acoustic Doppler current profilers on pilings measure velocity; with channel area and stage, they give discharge. "Robot boats" arrived in 2008, and stations can now be repaired or reset remotely. Data go out by cell modem within minutes, and many stations also measure salinity, turbidity, contaminants, chlorophyll and nutrients — linking water quality to flow.

Hydroacoustic instruments used in the network. USGS.

What the stations measure

• Delta outflow into San Francisco Bay (Rio Vista, Three Mile Slough, Jersey Point, Dutch Slough) — a key ecosystem measure.
• Delta transfer flow into the central delta, used by the State Water Project and Central Valley Project to meet salinity standards.
• Old and Middle River flow toward the export pumps, whose 14-day average is used to comply with court rulings and Endangered Species Act opinions.
• Sutter and Steamboat Sloughs, carrying half the water passing Sacramento at high flow and important for salmon migration.
• Yolo Bypass inflow and the Liberty Island–Cache Slough area, slated for big restoration and one of the few year-round homes of delta smelt.
• The Mokelumne system, where salmon survival is lower, and San Joaquin–central delta exchanges that affect salinity, turbidity and salmon.
• Export flows into the federal and state pumping plants.

Electronics inside a flow-station gage house. USGS.

Why it matters

• Public data: from the USGS National Water Information System and the California Data Exchange Center.
• Models: the data calibrate computer models of water levels, flows and salinity in the delta and bay.
• Science: sediment transport, the low-salinity zone, Chinook salmon and delta smelt migration, and CALFED adaptive-management experiments.
• Future changes: flooding Sherman Island or building the proposed twin tunnels could change tides, salinity and residence times across the delta — longer residence can fuel algae such as toxic Microcystis. The network documents conditions before and after.
• Next steps: adding real-time sensors for nutrients, phytoplankton, zooplankton and more, to link flows with habitats and species, and to respond to droughts, floods, earthquakes and sea-level rise.

The network has been supported over the years by the State Water Resources Control Board, the California Department of Water Resources, the U.S. Bureau of Reclamation, the Interagency Ecological Program, Stockton, Sacramento County and the Contra Costa Water District.

Sources

Based on Innovation in Monitoring: The U.S. Geological Survey Sacramento–San Joaquin River Delta, California, Flow-Station Network, USGS Fact Sheet 2015–3061, California Water Science Center, U.S. Geological Survey (DOI); a work of the United States government in the public domain. The map, photographs and diagrams come from the fact sheet's PDF; an aerial photograph from the California Department of Water Resources is left out.