Measuring the Colorado River at Lees Ferry

The Lees Ferry streamgage in September 1923. Photograph by G.C. Stevens, U.S. Geological Survey.

In 1921 the U.S. Geological Survey (USGS) put a streamgage on the Colorado River at Lees Ferry, Arizona, to track the river's flow and level as it enters Grand Canyon. The next year, the seven Colorado River Basin states — Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming — negotiated the 1922 Colorado River Compact to divide the river's water. It split the basin into an Upper Basin and a Lower Basin, with the dividing point about a mile below Lees Ferry, just downstream of where the Paria River joins the Colorado.

Why this gage matters

The Lees Ferry gage (USGS station 09380000) is one of the most important in the country because it measures how much water passes from the Upper to the Lower Basin through Glen Canyon Dam. The dam, built between 1956 and 1966, generates hydropower and stores water in Lake Powell to supply each state's share under the compact; the Lower Basin states depend on its releases. The gage, less than 16 miles below the dam, publishes real-time data on the river's flow.

The Upper and Lower Basins of the Colorado River, the Lees Ferry and Glen Canyon Dam area, and the basin's location. USGS figure.

In most years the Colorado runs dry before reaching its old outlet at the Gulf of California in Mexico, so measuring it at Lees Ferry matters to the Lower Basin's ecosystems, farms, cities and industries. Grand Canyon river guides and boaters use the water-level data to decide when to run rapids and where to camp on sandbars, and flow and water-quality data help track the health of fish, aquatic insects and other life that need particular conditions to survive and spawn. The USGS Arizona Water Science Center maintains the gage and collects its data.

USGS technicians collect water samples, prepare them on site and measure discharge at Lees Ferry. USGS photographs.

Measuring water level

The USGS runs more than 13,500 stations nationwide reporting real-time data on streams, lakes, reservoirs, groundwater and precipitation. A streamgage measures gage height — the water level above a fixed reference point called the gage datum, usually set below the streambed and unique to each site. Gage height is not water depth: a reading of 8 feet means the water is 8 feet above the datum, not that the river is 8 feet deep. At Lees Ferry a float in a stilling well and a radar system track the river's level every 15 minutes, and the readings go to the USGS Water Data for the Nation site every hour.

One common way to measure gage height: a stilling well and float. USGS figure.

Measuring flow

Discharge is the volume of water moving past a point per unit of time, such as cubic feet per second (cfs). Hydrologists measure it across a range of water levels to build a rating curve, which converts each gage-height reading into discharge in real time. Because channels keep changing — plants grow, sediment piles up, banks erode — the curve must be updated with fresh measurements.

At Lees Ferry, discharge is measured by moving an acoustic Doppler current profiler (ADCP) across the river by boat or cableway. It sends sound pulses into the water and reads the frequency shift of echoes off the riverbed and suspended particles to measure depth and velocity, and from those, discharge. Several passes are averaged to update the rating curve.

A boat-mounted ADCP measuring discharge. USGS figure.

An accurate curve is essential for understanding the dam's effect on the river. Daily release cycles — set by power demand, water availability and the Lower Basin's needs — can shift the water level by more than a foot and discharge by several thousand cfs in a day, sending a tide-like pulse through Grand Canyon. Peak flows often exceed 10,000 cfs.

Discharge at Lees Ferry, June 20–27, 2025, rising and falling each day with dam releases. USGS figure.

Water quality

Water quality has been sampled at Lees Ferry since the 1920s. Today it is part of the National Water Quality Network, established in 2013 to build long-term, comparable records at river sites nationwide on the foundation of earlier monitoring programs. Samples, collected across the channel from a crane-mounted boat and processed in a mobile laboratory before going to the USGS National Water Quality Laboratory, are tested for major ions such as sodium, chloride and calcium; trace metals such as uranium, arsenic and lead; nutrients such as nitrogen and phosphorus; pesticides; and suspended sediment.

Individual samples show trends but only a snapshot in time. Continuous meters measure pH, specific conductance, dissolved oxygen, temperature and turbidity in real time, for decisions about drinking water, regulation and recreation that cannot wait. At Lees Ferry the meter used to sit on the gage's stone housing on the bank, but over time its readings stopped reflecting the main channel. In February 2024 a buoy carrying a continuous meter was placed mid-channel near the gage, and its data are public on Water Data for the Nation.

The Colorado River at Lees Ferry, with the streamgage at the foot of the cliffs on the left. USGS photograph.

Arizona Water Science Center technicians at Lees Ferry. USGS photograph.

Real-time discharge at Lees Ferry is on Water Data for the Nation.

Sources

• U.S. Geological Survey Fact Sheet 2026–3002: "Arizona Water Science Center activities at Lees Ferry, Arizona."