Hydrologists measure how much water is in Washington's rivers and streams. The information is used to help manage water supplies, releases, and diversions in support of fish, farms, and domestic water needs across the state.
A basic description of the way streamflow is calculated can be found below.
Standard Operating Procedure for Measuring and Calculating Stream Discharge - EAP056 is the official document describing streamflow methods in detail.
Scientists visit streamflow monitoring sites about once every six weeks. They measure flow to confirm rating predictions or further develop the rating curve. In most conditions they measure streamflow with hydroacoustic equipment. These instruments include an Acoustic Doppler Velocimeter, called an ADV or an Acoustic Doppler Current Profiler, known as an ADCP. Typically used in smaller streams, ADVs bounce pulses of sound against moving particles in the stream to measure velocity. The ADCP is moved across larger streams on a raft. The ADCP measures stream depth and water velocity and almost instantaneously calculates streamflow. The change in the frequency of reflected sound (the "Doppler Effect") allows these instruments to calculate velocity and streamflow very accurately.
Hydrologists also perform maintenance or repairs on the equipment. They confirm the stage to maintain calibration of the data logger using a physical index that may include a staff gage, usually one or more porcelain enameled steel plates mounted to a secure structure; a wire-weight gage attached to a bridge, or a laser level mounted at a fixed elevation. Most stations are equipped with secondary or backup gage indices.
In addition, with the use of survey equipment, hydrologists periodically confirm the relative elevations of the physical stage indices (staff gages, wire weight gages, etc.) and determine if movement of these gages has occurred over time.
Findings from each visit to a streamflow gaging site, including any changes in the rating curve, are reported in the technical notes for that site, published with the streamflow site data and available in a tab on each flow monitoring network page.
Streamflow is the amount of water passing a specific point in a stream at a given time. It is sometimes called discharge, and it is often expressed in cubic feet per second, or cfs.
To speak scientifically about the water level in a river or stream, a hydrologist talks about a stream's "stage." The stage of a stream is its height or depth in relation to a fixed measuring point in the stream.
A "rating curve" is a mathematical tool used to predict streamflow based on stage height. Hydrologists develop a rating curve for a site after recording a series of area and velocity measurements of the stream at different stages. This allows them to predict streamflow with only stage height measurements. This rating curve is different for each gaging site and can change over time. Some stream channels are more stable than others, so some streams have more- or less-stable rating curves. Flow predictions based on more stable rating curves are generally considered more reliable.
Our gaging stations are generally equipped with instruments that measure and record a stream's stage by measuring water pressure. A gas-bubbler system or an electronic transducer at a fixed location below the water measures the pressure of the water above it. This measured pressure determines the stage of the water in the stream.
Some stations use radar gages, typically mounted on a bridge to determine stage. These instruments measure the rise and fall of stage relative to the radar gage's fixed location above the water surface.
A data logger records stage measurements every 15 minutes and sends this information to us via satellite. After the rating curve is applied to the stage record, the public can access streamflow information directly by going to each site on its Freshwater DataStream page.