U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2015–3061

January 2016

Innovation in Monitoring: 

The U.S. Geological Survey Sacramento–San Joaquin River 

Delta, California, Flow-Station Network 

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20153061

The U.S. Geological Survey (USGS) 

installed the first gage to measure the flow 

of water into California’s Sacramento–San 

Joaquin River Delta (figs. 1, 2) from the 

Sacramento River in the late 1800s. Today, 

a network of 35 hydro-acoustic meters 

measure flow throughout the delta. This 

region is a critical part of California’s 

freshwater supply and conveyance system. 

With the data provided by this flow-station 

network—sampled every 15 minutes and 

updated to the web every hour—state 

and federal water managers make daily 

decisions about how much freshwater 

can be pumped for human use, at which 

locations, and when. Fish and wildlife 

scientists, working with water managers, 

also use this information to protect fish 

species affected by pumping and loss of 

habitat. The data are also used to help 

determine the success or failure of efforts 

to restore ecosystem processes in what has 

been called the “most managed and highly 

altered” watershed in the country.

Understanding flow would be less 

challenging if managers and scientists 

were only trying to measure the outflow 

of the watershed’s myriad rivers and 

streams into San Francisco Bay and the 

ocean. However, the delta also receives a 

large amount of seawater that exchanges 

with the Pacific Ocean; peak tidal flows 

of 1.7 million cubic feet per second are 

exchanged into and out of the estuary by 

the tides through the Golden Gate. Twice 

every day, tides surge into the delta, causing 

reversed flows as far upstream as Freeport, 

less than 15 miles downstream from the 

city of Sacramento, before ebbing back to 

the ocean. It is a challenge to measure the 

amount of fresh water flowing through the 

delta’s labyrinth of channels because it is 

Delta outflow

San Joaquin

River/central

delta exchanges

Old and Middle

River flows

Yolo Bypass flow

Sutter/

Steamboat

corridor

Delta

transfer

flow

Mokelumne

River system

exchange

Export flows

Franks Tract

Yolo Bypass

Threemile Slough

Grant Line Canal

Sacramento Deep Water Ship

River

Mokelumne

Dry

Elk Slough

Steamboat

Slough

Sutter

Slough

Creek

San

Old

Middle

River

River

Joaquin

River

Channel

Sacramento

Rive r

Cosumnes River

State Water Project

pumping plant

Central Valley Project

pumping plant

SRV

SJJ

TSL

GES

GSS

DLC

FPT

SSS SDC

DSJ

MDM OBI

OH4

GLC

ODM

SJG

RYI

HWB

VCU

TRN

SUT

SMR NMR

LPS

MOK

OSJ PRI

HOL

ORO

SDI

MAL

LIB

DWS

HLT

FAL

Lodi

Walnut

Grove

Rio Vista

Hood

Freeport

Stockton

Middle River

Tracy

Pittsburg

Antioch

EXPLANATION

DMC Flow station and

 identifier

Pumping plant

Station groups

Yolo Bypass

Base modified from U.S. Geological Survey and other 

Federal and State digital data, various scale

Albers Equal Area Projection

North American Datum of 1983

121 ̊30' 121 ̊15'

37 ̊

45'

38 ̊

38 ̊

15'

38 ̊

30'

121 ̊45'

0 4 8 Miles

0 4 8 Kilometers

San Francisco

Sacramento River

San

Joaquin

River

Sacramento SacramentoSan Joaquin Delta

San 

Francisco

Bay

sac14-0528_fig 01

Figure 1. The Sacramento–San Joaquin River Delta showing the location of U.S. Geological 

Survey-operated sites.

so strongly influenced by tides. Yet in the last few decades, the 

USGS Bay–Delta hydrodynamics team has measured delta flows 

successfully (fig. 3) by pioneering techniques for improved 

measurements of tidal currents and freshwater inflows (Simpson 

and Oltmann, 1993; Simpson and Bland, 2000; Simpson, 2002; 

Ruhl and Simpson, 2005; Dinehart and Burau, 2005a, 2005b) to 

reflect real-time conditions in the delta more accurately.

The modern-day flow station employs technology 

similar to sonar, with hydro-acoustic devices (fig. 4) and other 

monitoring tools that are attached to pilings on the edges of delta 

channels. Measurements from these stations are telemetered to 

computers through cell-phone modems, which upload the data 

to the Internet within minutes (Sauer and Turnipseed, 2010). 

Many stations include sensors for measurement of components 
of water quality such as salinity, turbidity, contaminants, 

chlorophyll, and nutrients. By combining water-quality related 

data with hydrodynamic conditions—measured at the same 

time, place, and frequency—a more complete understanding 

of changes in the delta’s physical, chemical, and biological 

environment is emerging.

The data generated by the delta flow-station network are 

available not only to water managers, regulators, and scientists, 

but also to the public, from the National Water Information 

System (NWIS; http://waterdata.usgs.gov/nwis), where real-time 

data and data that have gone through quality-assurance checks 

are both available, and from the California Data Exchange 

Center (CDEC; http://cdec.water.ca.gov/), which contains 

real-time data only. Indeed, with this sophisticated flow-station 

network, the effects of existing dams, canals, and other delta 

modifications on flow patterns and water quality can be assessed. 

The flow-station network is a foundation for understanding the 

Sacramento–San Joaquin River Delta water-management system 

because of the importance of flow on many ecosystem processes 

and aquatic resources.

Measuring Net Flows in a Tidal Environment 

The Sacramento–San Joaquin River Delta is a tidal system 

in which water, and the organisms and constituents in the water, 

are in constant motion. The water can move rapidly, more than 

2 miles per hour, and travel long distances over each 12-hour 

flood-ebb tidal cycle. For example, a water parcel residing on 

Liberty Island (station LIB) at high tide can end up downstream 

from the Rio Vista Bridge (8–9 miles down estuary, station SRV) 

in 6 hours with an outgoing tide. 

Figure 2. Aerial view of Sacramento–San Joaquin River Delta. Photo 

courtesty of California Department of Water Resources.

Figure 3. Suite of hydroacoustic instrumentation used to monitor 

flow in the flow-station network. A, Deployment of an upward-looking 

acoustic Doppler current profiler (UL-ADCP) from a small research 

vessel. B, Schematic of a river cross section showing three different 

hydroacoustic technologies that were used in the flow-station network: 

(1) an ultrasonic velocity meter (UVM, a time of travel device no longer 

in commission); (2) an UL-ADCP; (3) a sideward-looking acoustic Doppler 

current profiler (SL-ADCP), which is currently in use. C, An UL-ADCP in a 

trawl-resistant mount. 

A

Acoustic path (UVM)

(2) UL-ADCP

(1) UVM

(3) SL-ADCP

B

UL-ADCP

C

Figure 4. Typical modern-day flow-station configuration for a 

sideward-looking acoustic Doppler current profiler (SL-ADCP). A, Aerial 

view of river with a SL-ADCP affixed to a piling, such that acoustic 

beams project across the river. B, Cross section of a river channel with 

the SL-ADCP affixed to a piling, such that acoustic beams project across 

the channel. C, A typical flow station set above the water, with gage 

house and solar panel. D, An SL-ADCP.

C

D

Water surfaceelevation gage

Sideward

looking

ADVM

Streamflow y-component

x-component

Beam angle

Echoes

Pings

Velocity measured

along acoustic

beam

A

B
The density of flow stations in the network is necessitated, 

in part, by the dynamic nature of transport processes in the Sacramento–San Joaquin River Delta. Calculating net flows (freshwater discharge) by using measured tidal flows is a classic signal 

(net flows) to noise (periodic tidal flows) problem. Because the 

net flows are small and tidal flows large, calculating the net flow 

places rigorous demands on the accuracy of tidal-flow estimates 

and every aspect of the data-collection process. 

For example, at the San Joaquin River at Jersey Point 

station (SJJ), the daily peak tidal flows can be on the order of 

150,000 cubic feet per second (cfs), whereas the net flows can 

be 2,000 cfs or less. Thus, estimates of net flows require that the 

tidal flow data be accurate to better than 1 percent, a challenging 

requirement. Even a small bias, on a percentage basis, in either 

the flood or ebb tidal discharge estimates can result in erroneous 

calculations of net flow, possibly in the wrong direction. 

Network History 

The flow-station network developed over time to address 

a series of specific questions. The first question, how much 

freshwater was flowing into the Sacramento–San Joaquin 

River Delta from the Sacramento River, was answered with the 

installation of the first hydro-acoustic meter at Freeport (Station FPT in fig. 1; see table 1 for NWIS station numbers) in 

1978. A decade later, water managers and scientists wanted to 

quantify the influence of water-export facilities on the northto-south movement of water from central to southern regions 

of the delta. In response, the USGS installed two more acoustic 

velocity meters at Old River (station OBI) by Bacon Island and 

at Middle River near Holt (HLT, 1987). In the early 1990s, the 

USGS installed two stations in the Walnut Grove area, so waterproject operators could find out how much water was flowing 

from the Sacramento River into the central delta through the 

Delta Cross Channel (DLC) and Georgiana Slough (GSS), the 

so-called delta transfer flow. Finally, a combination of four stations in the south delta, just upstream of the export pumps, was 

installed to estimate delta export flows (stations Old River near 

Byron, OH4; Victoria Canal, VCU; Great Line Canal, GLC; and 

Delta Mendota Canal, ODM). As of November 2015, the USGS 

operated 35 stations throughout the Sacramento–San Joaquin 

River Delta. 

Technological Advances 

Until the 1970s, getting just a snapshot of the net flows 

required campaigns over 30-hour periods employing multiple 

boats, multiple current meters (Price AA1), and long tag lines 

stretched across wide channels. It was difficult to routinely 

collect a time series of the net flows, the type of data that 

managers now take for granted. Today, most of the flow stations 

employ sideward-looking acoustic Doppler current profilers 

(SL-ADCPs) attached to pilings or channel markers (fig. 4). 

Measurements of velocity, cross-sectional area, and stage are 

used to compute volumetric discharge (for example, cubic feet 

of water per second). In 2008, “robot boats,” unmanned vessels 

equipped with acoustic devices, were introduced. More recently, 

new systems have been set up that enable scientists to repair 

or reset stations remotely (fig. 5). Such improvements greatly 

reduce the costs of running the flow-station network.

Table 1. Delta flow-station network. 

[Abbreviations: CDEC, California Data Exchange Center; NWIS, National Water Information System]

Name (CDEC abbreviation)

NWIS

station 

number

Name (CDEC abbreviation)

NWIS

station 

number

Sacramento River at Freeport CA (FPT) 11447650 San Joaquin River at Jersey Point (SJJ) 11337190

Sutter Slough at Courtland CA (SUT) 11447830 False River near Oakley CA (FAL) 11313440

Steamboat Slough near Walnut Grove CA (SSS) 11447850 Old River at Franks Tract near Terminous CA (OSJ) 11313452

Miner Slough at Hwy 84 bridge CA (HWB) 11455165 San Joaquin River at Prisoners Point near Terminous CA (PRI) 11313460

Sacramento River Deep Water Ship Channel near Rio Vista CA 

(DWS)

11455335 Middle River near Holt CA (HLT) 11312685

Cache Slough at south Liberty Island near Rio Vista CA (LIB) 11455315 Old River at Quimby Island near Bethel Island CA (ORQ) 11313434

Sacramento R at Delta Cross Channel CA (SDC) 11447890 Holland Cut near Bethel Island CA (HOL) 11313431

Delta Cross Channel near Walnut Grove CA (DLC) 11336600 Dutch Slough below Jersey Island Road at Jersey Island CA 

(DSJ)

11313433

Sacramento River below Georgiana Slough CA (GES) 11447905 Turner Cut near Holt CA (TRN) 11311300

Georgiana Slough near Sacramento River CA (GSS) 11447903 Old River near Bacon Island CA (OBI) 11313405

South Mokelumne River at New Hope bridge near Walnut 

Grove CA (SMR)

11336680 Middle River at Middle River CA (MDM) 11312676

North Mokelumne River near Walnut Grove CA (NMR) 11336685 San Joaquin River below Garwood bridge at Stockton CA (SJG) 11304810

Mokelumne River at Andrus Island near Terminous CA (MOK) 11336930 Old River near Byron CA (OH4) 11313315

Little Potato Slough at Terminous CA (LPS) 11336790 Victoria Canal near Byron CA (VCU) 11312672

Cache Slough at Ryer Island CA (RYI) 11455350 Grant Line Canal near Tracy CA (GLC) 11313240

Sacramento River at Rio Vista CA (SRV) 11455420 Old River at Delta Mendota Canal CA (ODM) 11312968

Three Mile Slough near Rio Vista CA (TSL) 11337080 Sacramento River at Mallard Island CA (MAL) 11185185

Sacramento River at Decker Island near Rio Vista CA (SDI) 11455478

1

Use of trade names in this manuscript is for descriptive purposes only and 

does not constitute endorsement by the U.S. Geological Survey.
as OMR (Old and Middle River flow), represents the flow of 

water toward the export facilities from the north. The data from 

these stations currently are used to adjust water management to 

comply with a variety of court decisions and biological opinions 

under the Endangered Species Act. The OMR flow parameter 

used for regulatory purposes is a 14-day average of the measured 

(tidal) flows.

Sutter-Steamboat Corridor 

 Sutter and Steamboat Sloughs are important conveyance 

channels that carry half of the water that flows past the city of 

Sacramento at high water. Sutter Slough carries the bulk of the 

net flow; Steamboat Slough is more tidally dominated. The flows 

in both of these channels are strongly influenced by Sacramento 

River flows and Delta Cross Channel gate operations. 

Hydrodynamic data collected at Sutter (SUT) and Steamboat 

(SSS) Sloughs are also important for the study of salmon outmigration (Perry and others, 2013; Romine and others, 2013). 

Yolo Bypass 

 The flows entering the Sacramento-San Joaquin Delta from 

the Yolo Bypass are computed by subtracting the flow in Miner 

Slough (HWB) from the flow in Cache Slough (RYI). This computation also defines the tidal and net exchanges into the Liberty 

Island/Cache Slough region, an area slated for large restoration 

efforts. Moreover, this region is one of the few places where 

delta smelt are found year round (Sommer and Mejia, 2013). 

Mokelumne River System Exchange 

 Most of the Sacramento River water that is exported 

flows through the Mokelumne River system. When the Delta 

Cross Channel gates are open, this region is essentially riverine. 

Conversely, when the gates are closed, this system is primarily 

tidal. The data from the Mokelumne (MOK) and Little Potato 

Slough (LPS) stations also are relevant to salmon migration, 

because salmon outmigrants that take this route have lower survival (Perry and others, 2013), and are critical for monitoring the 

system’s response to the proposed restoration of McCormack–

Williamson Tract and Staten Island.

San Joaquin River/Central Delta Exchanges 

 Exchanges of water from the San Joaquin River into the 

central delta are important for understanding how salinity and 

turbidity (from suspended sediments and organic matter) change 

in the system. The four stations used to calculate this exchange 

are Turner Cut (TRN), Middle River north of Mildred Island 

(HLT), Old River north of Frank’s Tract near the confluence of 

the San Joaquin and Mokelumne Rivers (OSJ), and False River 

(FAL). These exchanges strongly influence the rate of entrainment of San Joaquin River salmon outmigrants into the central 

delta (Holbrook and others, 2008; Buchanan and others, 2013). 

Export Flows 

 The volume of water entering the federal and state export 

facilities from the various “feeder” channels is calculated by 

using data from the following stations: Old River at Highway 

4 (OH4), Victoria Canal (VCU), Grant Line Canal (GLC), and 

Delta Mendota Canal (ODM).

Figure 5. Electronics inside a flow-station gage house.

Station Groups 

Scientists specifically selected the station locations to 

assess transport (the movement of water and everything in it) 

at a variety of spatial scales, from the individual channel to 

regional scales and to the Sacramento–San Joaquin River Delta 

as a whole. The stations are grouped, as shown in figure 1, to 

calculate and verify localized inputs and outputs of water and 

localized storage. The eight groups are described here. 

Delta Outflow 

 The sum of the measured flows from stations at Rio Vista 

(SRV), Three Mile Slough (TSL), San Joaquin River at Jersey 

Point (SJJ), and Dutch Slough (DSJ) are used to estimate delta 

outflow. Delta outflow is a key ecosystem metric because it is a 

measure of water flowing into San Francisco Bay (that is, inputs 

less exports and consumptive use within the delta). 

Delta Transfer Flow 

Delta transfer flow is the difference between the flows 

measured at stations in the Sacramento River upstream from the 

Delta Cross Channel (SDC) and the Sacramento River downstream from Georgiana Slough (GES). The calculation is used 

by water managers to estimate the amount of Sacramento River 

water that flows into the central delta through the Mokelumne 

system, including the Delta Cross Channel and Georgiana 

Slough. The water associated with the delta transfer flow is used 

by state (the State Water Project) and federal (Central Valley 

Project) water-project operators to meet salinity standards in the 

central delta. The delta transfer flow can now also be calculated 

by summing the flow in the Delta Cross Channel (DLC) and 

Georgiana Slough (GSS) directly.

 Old and Middle Rivers Flows 

 The sum of the flows at stations OBI and Middle River at 

Middle River (MDM), referred to by the regulatory agencies 
Uses of Network Data

Data from the network are used by scientists and by 

managers of the area who make decisions and plans for the 

future. The data are also used for calibration and validation 

of numerical computer models that can estimate water levels, 

flow speeds and discharges, and even the spatial and temporal 

evolution of salinity in the delta and San Francisco Bay 

(Resource Management Associates, 2005; MacWilliams and 

others, 2008; DSM2 User Group Portal, 2014). Measurements 

from the network are required input for models of this type. 

In addition, the flow-station data, and the USGS California 

Water Science Center hydrodynamics team, have played an 

important role in numerous interdisciplinary investigations of 

subjects such as sediment transport processes, the behavior 

of the low-salinity zone in the delta, and the outmigration of 

Chinook salmon and the upmigration of delta smelt. The flowstation network was the foundation for the process-based, largescale, adaptive-management experiments undertaken by the 

CALFED Science Program, and in the future, it could support 

the projects envisioned by the Delta Stewardship Council (the 

Delta Plan) and the Bay Delta Conservation Plan (BDCP). 

This network can provide the base-line data to help document 

the effects of proposed changes in conveyance (for example, 

the tunnels proposed as part of the Bay Delta Conservation 

Plan) and of restoration efforts in the future, for which the data 

collected now document baseline (pre-project) conditions.

Uncertainty about New Facilities and Habitats 

Decades of hydrodynamics monitoring, modeling, and 

special studies indicate that restoration or changes in water 

conveyance in one area can substantially affect basic hydrodynamic processes and transport in others. Many changes are 

proposed for the Sacramento–San Joaquin River Delta to meet 

the State’s goals of “providing a more reliable water supply for 

California and protecting, restoring, and enhancing the delta 

ecosystem” (Delta Stewardship Council, 2013). Documenting 

how these changes affect flows in the delta is important. The 

proposed flooding of Sherman Island, for example, could affect 

hydrodynamics and transport processes, including salinity intrusion, throughout the delta. Withdrawing water from the system 

into an isolated water-conveyance facility, such as the currently 

proposed twin tunnels, would also alter transport throughout 

the delta. If built, net flows throughout the north and western 

Sacramento–San Joaquin River Delta would be proportionately 

reduced by the amount withdrawn into the conveyance facility, 

increasing the influence of the tides throughout the delta. If the 

conveyance facility is built, the north-to-south draw of water 

across the delta that has existed for decades would likely be 

reduced as a result of compensatory reductions in pumping 

from the south delta, creating much longer average residence 

times. Longer residence times are associated with higher rates 

of algal growth, which could fuel eutrophication in some 

regions, including increased blooms of nuisance algae, such 

as Microcystis, which is toxic to humans and other organisms 

(Lehman and others, 2013). In the coming decades, the flowstation network can provide data that address uncertainty 

concerning the location of proposed water-conveyance facilities 

and that, after they are built, document the effects of these new 

water-conveyance facilities, management actions, and habitatrestoration efforts.

Climate Change and Endangered Species

The comprehensive coverage of the flow network provides 

valuable data for developing responses to any new endangered 

species listings and ecosystem changes, as well as to flooding, 

droughts, and hydrodynamic changes due to earthquakes, 

storms, and sea-level rise. 

Taking the Network to the Next Level 

 The dynamic nature of the delta system makes it very 

difficult to understand the transport and fate of constituents. 

Indeed, efforts are currently being made to add technology 

to existing flow stations that can measure and report in real 

time a broad suite of physical, optical, particle, and waterquality parameters. This multi-analytical approach allows for 

better integration of information across disciplines and for 

more powerful diagnostic tools. It allows scientists and water 

managers to continuously characterize parameters related to 

nutrient uptake, phytoplankton community structure, and zooplankton and fish-foraging efficiency at the same time as more 

common water-quality information (for example, dissolved 

oxygen, nutrients, and pH) in the context of the existing flow 

and turbidity monitoring network. When further coordinated 

with monitoring of biological organisms, such as fish, birds, 

and invasive species, these simultaneous data sets can also 

help scientists gain a better understanding of the influences of 

flows on the extent of estuarine and riparian habitats and on the 

distribution of species in the Sacramento–San Joaquin River 

Delta. 

Acknowledgements

The flow-station network would not have been possible 

were it not for the generous support, throughout the years, of 

a variety of agencies and individuals. Jerry Johns, during his 

tenure with the State Water Resources Control Board, provided 

the initial funding for the installation of the acoustic velocity 

meter (AVM) at Freeport and funded the installation and initial 

operation of ultrasonic velocity meters (UVMs) at Old and 

Middle Rivers. Randy Brown, with the California Department 

of Water Resources (DWR), and Ken Lentz, with the U.S. 

Bureau of Reclamation (USBR), provided much of the support for the development of acoustic technologies to measure 

tidal flows in the delta and supported the operation of the flow 

network. The Interagency Ecological Program (IEP) continues 

to support the flow-station network substantially, and we thank 

Ted Sommer, Karen Gehrts, and others for their ongoing support. The conveyance branch at the California Department of 

Water Resources (DWR) funded the installation, operation, 

and maintenance of 16 sites in the north and central delta for a 

period of 5 years under the auspices of the Franks Tract Project 

until bond funds were cut as a result of the State’s fiscal crisis. 

The USBR, through Erwin Van Nieuwenhuyse and others, took 

over support of these stations. Finally, support from the city 

of Stockton, Sacramento County and the Contra Costa Water 

District (CCWD) over the years is gratefully acknowledged.
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Additional Information and Contacts

USGS California Water Science Center

6000 J Street, Placer Hall

Sacramento, California 95819-6129

http://ca.water.usgs.gov/topic.html?topic=HST

USGS Data Repository 

(National Water Information System, NWIS)

http://waterdata.usgs.gov/ca/nwis/

California Data Exchange Center (CDEC)

Delta flow stations

http://www.cdec.water.ca.gov/

Jon Burau, jrburau@usgs.gov

Cathy Ruhl, cruhl@usgs.gov

Paul Work, pwork@usgs.gov