AzWSC Capabilities: Reach-Scale Monitoring

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For over 125 years, the U.S. Geological Survey (USGS) streamgage network has provided important hydrologic information about rivers and streams throughout the Nation. Traditional streamgage methods provide reliable stage and streamflow data but typically only monitor stage at a single location in a river and require frequent calibration streamflow measurements. Direct measurements are not always feasible, therefore improved sensors and methods are being deployed at gages to better document streamflow conditions between measurements. The technology and techniques of reach-scale monitoring allow the U.S. Geological Survey to collect more data across the full range of streamflow without requiring that a hydrographer be present. The Arizona Water Science Center’s reach-scale monitoring program enhances the Arizona streamgage network with more accurate streamflow measurements and provides more extensive streamflow records and geomorphological datasets for our agency partners and the public. Reach-scale monitoring installations and techniques are applicable to streams of the western United States and likely throughout the Nation.

Floods in the United States are extremely dangerous, costly, and routinely threaten life, property, and infrastructure. The USGS streamgage network quantifies the Nation’s water supply, provides data for flood warning and forecasting, informs critical infrastructure decisions, and delivers valuable hydrologic information to better understand the responses of rivers and streams to changes in their watersheds. However, traditional streamgages typically only monitor stage at a single location in a river and require frequent calibration streamflow measurements by hydrographers to maintain accuracy. Direct streamflow measurements can prove difficult or impossible, especially in the arid southwest, where streamflow can be infrequent and flashy, sites may be remote or inaccessible during high streamflow, infrastructure from which to measure floods is limited, and conditions may pose hazards to field personnel.

Reach-scale monitoring employs recent advances in techniques and technology to develop a better understanding of the hydraulics of a river reach. At a reach-scale monitoring installation, compact pressure transducers, video cameras, velocity radars, tilt sensors, light detection and ranging (lidar)scanners, and small unmanned aircraft systems (sUAS), in conjunction with traditional monitoring methods, account for more hydrologic parameters over larger areas of a stream compared to a traditional streamgage. Reach-scale monitoring gages consist of an array of sensors deployed  throughout the river reach to remotely and continuously measure river stage and velocity. Complete topographic models of the river reach are obtained with GPS, lidar, or photogrammetric surveys. With channel geometry data in hand, velocity data can be collected during streamflow eventsusing radar sensors, video cameras, and image velocimetry technology to compute streamflow. These robust datasets provide measurements in previously unmeasurable conditions and during every streamflow event, potentially improving traditional streamgage accuracy. Reach-scale monitoring gages provide data necessary to monitor changes in channel geometry over time and collect parameters needed to build an accurate hydraulic model—that is, a numerical streamflow simulation that represents the direction and magnitude of streamflow in the river channel. Reach-scale monitoring methods allow the USGS to collect more data across the full range of streamflow and can generally do so without requiring that a hydrographer be present. These new methods provide more accurate, safer, and efficient ways to measure streamflow.

See “Reach-scale monitoring and modeling of rivers—Expanding hydraulic data collection beyond the cross section” for more information about reach-scale monitoring.