How Does the USGS Collect Streamflow Data?

Science Center Objects

The USGS follows four basic steps for collecting streamflow data. The following example uses a streamgage though more portable methods are also frequently used. Read on to learn more.

•  Water Science School HOME  •  Surface Water topics  •

USGS Gage 13011500 PACIFIC CREEK AT MORAN WY  With River During Winter

USGS gage 13011500 PACIFIC CREEK AT MORAN WY from the Dakota Water Science Center during winter 2017 with River in background. (Credit: Steve Shivers, USGS)

1.  A Gage Site is Established

The U.S. Geological Survey selects a suitable site along a river or stream and constructs a gage house to hold equipment that measures and records the height of the water surface (gage height or stage).  The gage house also can hold equipment that measures water-quality parameters, such as temperature, pH, dissolved oxygen, and dissolved chemicals, and weather conditions, such as air temperature, precipitation, and wind speed.

 

USGS streamgage diagram

USGS streamgage diagram

2. The Water Level is Measured and Recorded

The gage height (or stage) of a river most commonly is measured through the use of a stilling well, a bubble system, or a wire-weight gage.

A stilling well is used when a gage house can be built immediately adjacent to a river.  The well is connected to the stream with pipes so that when the water level in the stream changes, the water level in the well also changes.  A float in the well is connected to a recorder or data-collection platform.

Diagram of a streamgage installation, USGS

Bubble system.

The bubble system can be used when construction of a well is not feasible.  The bubble system requires a long open-ended pipe that extends from the gage house to the river.  One end of the pipe is fixed securely below the water surface, and pressurized gas (usually nitrogen or air) is forced through the pipe from inside the gage house and out a submerged opening called an orifice.  The pressure in the pipe is determined by how deep the water is over the orifice.  A change in the water level of the river produces a corresponding change in the pressure in the pipe.  The change in pressure is converted to an electronic signal by a transducer inside the gage house.  Data from the transducer then is fed to a recorder, or data-collection platform, which records the corresponding gage height.

An outside reference gage, typically a vertical graduated ruler called a staff gage, is read periodically to verify that the recorded gage heights from the stilling well or bubble system are the same as the water levels in the stream.

A wire-weight gage consists of a drum wound with a single layer of stainless-steel cable attached to a bronze weight, a graduated disc, and a counter, all within a cast-aluminum box.  The weight may be lowered to the water surface using a hand crank.  When the bottom of the weight is at the water surface, the gage height is indicated by the combined readings of the counter and the graduated disc.  The wire-weight gage commonly is mounted on a bridge handrail, parapet wall, or pier for use as an outside gage.

 

Image: USGS Current Meter at the Bonnet Carré Spillway

A current meter. The rotating silver cups measure how fast the water is flowing. Meanwhile, the black torpedo-shaped object is a weight that keeps the current meter relatively stationary and the fins keep the current meter facing into the approaching flow. (Credit: Alex Demas, USGS)

3. Discharge and Velocity are Measured

The volume of water passing a specific point in a given interval of time is called discharge.  Discharge generally is measured in cubic feet per second and most often is determined by making measurements of a particular cross-section area of the river and of how fast the water is flowing per second (velocity) past that cross section.  Discharge then is calculated by multiplying the width, depth, and velocity of that section of the river.

Velocity is measured by using a current meter (left).  The meter consists of cups that are rotated by the action of flowing water.  The speed of the rotation depends on the velocity of the water passing by the cups.  A technician counts the number of revolutions in a given time interval and determines the velocity at the location of the meter.  The stream is divided into segments and the average velocity and depth of the stream are measured at each segment.  The discharges in each of the segments then are summed to obtain the total stream discharge.  Technicians obtain measurements in several ways, depending on the size and depth of the river.  They will wade across small streams, use suspended cableway cars, stand on bridges, or work out of boats to measure discharge in large rivers.

 

Rating Curve

Stage-discharge relation graph.

4.  A Relation Between Water Level and Discharge is Developed

A stage-discharge relation or rating curve is used to relate water level to associated discharge.  The rating curve for a specific stream location is developed by making successive discharge measurements to define and maintain a stage-discharge relation.  These discharge measurements and their corresponding stages (or gage height) then are plotted on a graph for each station.  Continuous discharge throughout the year can be determined from the rating curve and the record of river stage (gage height).  Factors such as ice cover, debris, and vegetation growth can affect the stage-discharge relation, and the data must be checked periodically to ensure accuracy.