Water-Quality Super Gages in Indiana and Kentucky

A USGS super gage at Davis, Indiana, which measures, records and transmits water quality continuously. USGS.

What a super gage is

A water-quality super gage (or sentry gage) at a USGS streamgage measures streamflow and water quality continuously, day and night, all year, and posts the data online in near real time. Occasional samples can miss changes that happen at night, during storms, or when a sampling crew can't get there.

How it works

• Flow: an instrument records stream stage, which routine velocity measurements convert into streamflow. Some constituents rise with runoff in high flows; others are diluted.
• Water quality: an in-stream sonde measures turbidity, dissolved oxygen, specific conductance, pH and temperature, usually every 15 minutes (as often as every 5). Some gages add an ultraviolet nitrate sensor and a phosphate analyzer. A data logger stores readings, and satellite telemetry sends them to the web hourly.
• Quality control: crews regularly clean and recalibrate instruments, measure drift from biofouling, and check against independent meters and standard solutions.
• Stream samples: depth- and width-integrated samples go to a laboratory to check the sensors.
• Surrogate models: statistical regressions link lab concentrations to sensor readings, so hard-to-measure constituents can be computed continuously — turbidity for suspended sediment, the nitrate sensor for total nitrogen, the phosphate analyzer (or turbidity plus flow) for total phosphorus. Loads — concentration × flow — are computed every 15 minutes and summed into daily, monthly and annual loads and yields per square mile.

One of the instruments immersed in the stream at a super gage. USGS.

Three projects

Lower Green River, Kentucky. A heavily agricultural basin chosen for the Mississippi River Basin Healthy Watersheds Initiative, led by the Natural Resources Conservation Service, which aims to improve water quality while keeping farms productive. Since 2013, the USGS and the Kentucky Governor's Office of Agricultural Policy have measured continuous nitrate and computed the nitrate loads the river carries to the Ohio River — the test of whether nutrient-management plans work.

Eagle Creek, Zionsville, Indiana. A demonstration and test site since 2010, north of Indianapolis, with water-quality instruments, a weather station, soil-moisture probes and a video camera, and a testbed for nitrate sensors. The town has more than tripled since 2000, turning farmland suburban. Eagle Creek feeds Eagle Creek Reservoir, an Indianapolis water source with episodic algal blooms.

Kankakee River, Indiana. The basin drains 2,989 square miles into the Illinois River; the Grand Kankakee Marsh was drained and many streams channelized for farming. Since 2012, with the Kankakee River Basin Commission, the USGS has measured turbidity and flow at three gages on the Yellow River to estimate sediment loads, and runs super gages on the Kankakee at Davis and the Iroquois at Foresman to track the effects of land management.

The Indiana and Kentucky sentry gages belong to a national network. USGS.

Getting the data

The National Real-Time Water Quality website has interactive state maps of continuous gages and the constituents they measure, maps of gages with surrogate models, the published methods, and model results as plots or tables with 90-percent confidence intervals. Every record is reviewed daily, verified against cross-stream samples and rated for quality.

Sources

Based on Megan E. Shoda, Timothy R. Lathrop and Martin R. Risch, "Real-Time, Continuous Water-Quality Monitoring in Indiana and Kentucky," U.S. Geological Survey fact sheet; a work of the United States government in the public domain. The photographs and map are taken from the fact sheet's PDF.